Control circuit and switch device

ABSTRACT

A control circuit controls a switching element including a gate and a source corresponding to the gate. The control circuit includes an inductor, a circuit element, and a resistor. The inductor is connected between the gate and the source of the switching element. The circuit element is connected in series to the inductor between the gate and the source. The circuit element allows an electric current to flow therethrough in response to generation of electromotive force in the inductor. The resistor is connected in parallel to the inductor and the circuit element between the gate and the source.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Patent Application No. PCT/JP2021/014530, filed on Apr. 5,2021, which in turn claims the benefit of Japanese Patent ApplicationNo. 2020-069303, filed on Apr. 7, 2020, and Japanese Patent ApplicationNo. 2020-077832, filed on Apr. 24, 2020, the entire disclosures of whichApplications are incorporated by reference herein.

TECHNICAL FIELD

The present disclosure generally relates to a control circuit and aswitch device, and more particularly relates to a control circuit forcontrolling a switching element and a switch device including such acontrol circuit.

BACKGROUND ART

Patent Literature 1 proposes a bidirectional switch circuit with theability to reduce overvoltage applied to a switching transistor.

In the exemplary bidirectional switch circuit disclosed in PatentLiterature 1, a reactor is inserted between the respective sources oftwo switching transistors. In addition, between the gate and source ofeach switching transistor, a diode is connected as an electromotiveforce supply element to have polarity that prevents a gate drive voltagefrom being applied to each switching transistor. A drive voltage for agate driver circuit is supplied via a series resistor to between a firstcontrol terminal that is connected to a common gate of the two switchingtransistors and a second control terminal connected to an intermediatetap of the reactor.

A control circuit for controlling a semiconductor switch (switchingelement) is sometimes required to reduce a surge voltage applied to thesemiconductor switch while cutting down the switching loss involved whenthe semiconductor switch turns OFF.

CITATION LIST Patent Literature

Patent Literature 1: JP H04-296116 A

SUMMARY OF INVENTION

To overcome such a problem, it is an object of the present disclosure toprovide a control circuit and a switch device that may be expected toreduce a surge voltage applied to a switching element while cutting downthe switching loss involved when the switching element turns OFF.

A control circuit according to an aspect of the present disclosure is acontrol circuit for controlling a switching element including a gate anda source corresponding to the gate. The control circuit includes aninductor, a circuit element, and a resistor. The inductor is connectedbetween the gate and the source of the switching element. The circuitelement is connected in series to the inductor between the gate and thesource. The circuit element allows an electric current to flowtherethrough in response to generation of electromotive force in theinductor. The resistor is connected in parallel to the inductor and thecircuit element between the gate and the source.

A switch device according to another aspect of the present disclosureincludes the control circuit described above and the switching elementdescribed above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of a switch device including a controlcircuit according to a first embodiment;

FIG. 2 illustrates how voltages and currents change with time atrespective points on the control circuit;

FIG. 3 is a graph illustrating how a source current flowing through aswitching element of the switch device including the control circuitchanges with time as its parameter is varied;

FIG. 4 is a circuit diagram of a switch device including a controlcircuit according to a first variation of the first embodiment;

FIG. 5 is a circuit diagram of a switch device including a controlcircuit according to a second variation of the first embodiment;

FIG. 6 is a graph illustrating how a source current flowing through aswitching element of the switch device changes with time as itsparameter is varied;

FIG. 7 is a circuit diagram of a switch device including a controlcircuit according to a third variation of the first embodiment;

FIG. 8 is a circuit diagram of a switch device including a controlcircuit according to a fourth variation of the first embodiment;

FIG. 9 is a circuit diagram of a switch device according to a secondembodiment;

FIG. 10 is a circuit diagram of a switch device according to a firstvariation of the second embodiment;

FIG. 11 is a circuit diagram of a switch device according to a secondvariation of the second embodiment;

FIG. 12 is a circuit diagram of a switch device including a controlcircuit according to a third embodiment;

FIG. 13 is a circuit diagram of a switch device including a controlcircuit according to a variation of the third embodiment;

FIG. 14 is a circuit diagram of a switch device including a controlcircuit according to a fourth embodiment;

FIG. 15 is a circuit diagram of a switch device including a controlcircuit according to a fifth embodiment;

FIG. 16 is a conceptual diagram of a switch system including a controlcircuit according to a first example;

FIG. 17 is a circuit diagram of a switch system including the controlcircuit;

FIG. 18 illustrates how the control circuit operates;

FIG. 19A illustrates how the control circuit operates;

FIG. 19B is a waveform chart showing how the control circuit operates;

FIG. 20A illustrates how the control circuit operates;

FIG. 20B is a waveform chart showing how the control circuit operates;

FIG. 21A illustrates how the control circuit operates;

FIG. 21B is a waveform chart showing how the control circuit operates;

FIG. 22 is a characteristic diagram of a switch system including thecontrol circuit;

FIG. 23 is a circuit diagram of a switch system including a controlcircuit according to a second example;

FIG. 24 is a circuit diagram of a switch system including a controlcircuit according to a third example;

FIG. 25 is a circuit diagram of a switch system including a controlcircuit according to a fourth example;

FIG. 26 is a circuit diagram of a switch system including a controlcircuit according to a fifth example;

FIG. 27 is a circuit diagram of a switch system including a controlcircuit according to a sixth example; and

FIG. 28 is a circuit diagram of a switch system including a controlcircuit according to a seventh example.

DESCRIPTION OF EMBODIMENTS First Embodiment

A control circuit 10 according to an exemplary embodiment and a switchdevice 100 including the control circuit 10 will be described withreference to FIGS. 1-3 .

(1) Overview

As shown in FIG. 1 , the control circuit 10 is a control circuit forcontrolling a switching element 1 including a gate G1 and a source S1corresponding to the gate G1. The switching element 1 includes not onlythe gate G1 and the source S1 but also a drain D1 as well. The controlcircuit 10 includes: an inductor L1 connected between the gate G1 andthe source S1 of the switching element 1; and a circuit element 5connected in series to the inductor L1 between the gate G1 and thesource S1 and turning electrically conductive in response to generationof electromotive force in the inductor L1. As used herein, if thecircuit element 5 “turns electrically conductive in response togeneration of electromotive force in the inductor L1,” this phrase meansthat an electric current flows through the circuit element 5 whenelectromotive force is generated in the inductor L1 to make thepotential at a second terminal, which is located opposite from a firstterminal connected to the source S1 of the switching element 1, higherthan the potential at the first terminal. In other words, if the circuitelement 5 “turns electrically conductive in response to generation ofelectromotive force in the inductor L1,” this phrase means that anelectric current flows through the circuit element 5 in response togeneration of counter electromotive force in the inductor L1. Thecontrol circuit 10 further includes a resistor R1 connected in parallelto the inductor L1 and the circuit element 5 between the gate G1 and thesource S1.

The inductor L1 generates electromotive force (induced electromotiveforce) in accordance with a current variation rate (di/dt=dIs/dt) of asource current Is that is a principal current of the switching element 1when the switching element 1 turns OFF. In this case, the source currentIs that is the principal current of the switching element 1 is anelectric current that flows from the drain D1 to the source S1 of theswitching element 1. That is to say, the source current Is is the samecurrent as the drain current.

The circuit element 5 allows an electric current to flow therethrough inresponse to generation of electromotive force in the inductor L1according to the current variation rate of the source current Is whenthe source current Is decreases. The circuit element 5 is, for example,a capacitor C1.

The resistor R1 is connected in parallel to the inductor L1 and thecircuit element 5. In other words, the resistor R1 is connected inparallel to a series circuit including the inductor L1 and the circuitelement 5. The control circuit 10 includes the resistor R1, andtherefore, may generate a potential difference between the terminals ofthe resistor R1. Thus, the control circuit 10 may make the referencepotential of the potential at the gate G1 of the switching element 1(i.e., gate potential) and the reference potential of the potential atthe source S1 of the switching element 1 (i.e., source potential)different from each other.

The switch device 100 includes the control circuit 10 and the switchingelement 1. In the switch device 100, a load circuit including a seriescircuit of a load and a power supply, for example, may be connectedbetween the drain D1 and source S1 of the switching element 1. Morespecifically, in the switch device 100, the load circuit including theload and the power supply may be connected between a first terminal,which is one terminal of a series circuit of the switching element 1 andthe inductor L1, and a second terminal, which is the other terminalthereof. Note that the load and the power supply are not constituentelements of the switch device 100.

(2) Configuration (2-1) Switching Element

The switching element 1 is, for example, a GaN-based semiconductorswitching element. More specifically, the switching element 1 is ajunction field effect transistor (JFET). The JFET serving as theswitching element 1 is, for example, a GaN-based gate injectiontransistor (GIT).

The switching element 1 includes, for example, a substrate, a bufferlayer, a first nitride semiconductor layer, a second nitridesemiconductor layer, a source electrode, a gate electrode, a drainelectrode, and a p-type layer. The buffer layer is formed on thesubstrate. The first nitride semiconductor layer is formed on the bufferlayer. The second nitride semiconductor layer is formed on the firstnitride semiconductor layer. The source electrode, the gate electrode,and the drain electrode are formed on the second nitride semiconductorlayer. The p-type layer is interposed between the gate electrode and thesecond nitride semiconductor layer. In the switching element 1, a diodestructure is formed by the second nitride semiconductor layer and thep-type layer. The gate G1 of the switching element 1 includes the gateelectrode and the p-type layer. The source S1 of the switching element 1includes the source electrode. The drain D1 of the switching element 1includes the drain electrode. The substrate is a silicon substrate, forexample. The buffer layer is an undoped GaN layer, for example. Thefirst nitride semiconductor layer is, for example, an undoped GaN layer.The second nitride semiconductor layer is, for example, an undoped AlGaNlayer. The p-type layer is, for example, a p-type AlGaN layer. Each ofthe buffer layer, the first nitride semiconductor layer, and the secondnitride semiconductor layer may include impurities such as Mg, H, Si, C,and O to be inevitably contained during their growing process bymetal-organic vapor phase epitaxy (MOVPE), for example.

(2-2) Switch Device

As shown in FIG. 1 , the switch device 100 includes the switchingelement 1, the control circuit 10, a drive circuit 2, and a driver 3.The control circuit 10 according to the first embodiment includes theinductor L1, the capacitor C1 serving as a circuit element 5, and theresistor R1 as described above.

The driver 3 has a high-potential output terminal and a low-potentialoutput terminal. In this switch device 100, the high-potential outputterminal of the driver 3 is connected to the gate G1 of the switchingelement 1 via the drive circuit 2. The drive circuit 2 includes, forexample, a gate resistor connected between the high-potential outputterminal of the driver 3 and the gate G1 of the switching element 1. Thelow-potential output terminal of the driver 3 is connected to the sourceS1 of the switching element 1 via the resistor R1. The driver 3 is adriver which may apply not only a positive bias voltage but also anegative bias voltage to between the gate G1 and source S1 of theswitching element 1. The driver 3 is a driver which includes, forexample, a DC power supply and a complementary metal-oxide semiconductor(CMOS) inverter and which may change the output voltage within the rangefrom −12 V to 18 V.

The source S1 of the switching element 1 is connected to a firstterminal of the inductor L1 and a first terminal of the resistor R1. Thefirst terminal of the resistor R1 is connected to a node N1 on the pathbetween the source S1 of the switching element 1 and the first terminalof the inductor L1. The gate G1 of the switching element 1 is connectedto the high-potential output terminal of the driver 3 via the drivecircuit 2. A first terminal of the capacitor C1 is connected to thesecond terminal of the inductor L1. The capacitor C1 is connected to anode N2 on the path between the inductor L1 and a second terminal to beconnected to the load circuit described above. The second terminal ofthe capacitor C1 is connected to a node N3 on the path between theresistor R1 and the gate G1 of the switching element 1. Morespecifically, the second terminal of the capacitor C1 is connected tothe second terminal of the resistor R1 and the low-potential outputterminal of the driver 3. The resistor R1 is connected in parallel tothe inductor L1 and the capacitor C1. That is to say, the resistor R1 isconnected in parallel to a series circuit of the inductor L1 and thecapacitor C1. It can be said that the node N3 is a point of connectionbetween the resistor R1 and the circuit element 5. In the followingdescription, an arbitrary point on the path between the node N3 and thelow-potential output terminal of the driver 3 will be hereinafterreferred to as a “reference potential point P0” and the potential at thereference potential point P0 will be hereinafter referred to as a“reference potential Vstd” for the sake of convenience of description.

(3) Operation

Next, it will be described with reference to FIGS. 1-3 how the switchdevice 100 operates.

In the following description, the voltage between the gate G1 and sourceS1 of the switching element 1 will be hereinafter referred to as a“gate-source voltage Vgs” and an electric current flowing from the gateG1 of the switching element 1 to the drive circuit 2 will be hereinafterreferred to as a “discharge current Idis.”

In the switch device 100, while a positive bias voltage is applied fromthe driver 3 to between the gate G1 and source S1 of the switchingelement 1 to make the gate-source voltage Vgs of the switching element 1equal to or higher than the threshold voltage of the switching element1, the switching element 1 is ON state. To turn the switching element 1OFF, the switch device 100 changes the output voltage of the driver 3from the positive bias voltage into 0 V (or a negative bias voltage),for example. As a result, in the switch device 100, the source currentIs, the electromotive force VL of the inductor L1, the gate-sourcevoltage Vgs, the reference potential Vstd, and the discharge currentIdis change as shown in FIG. 2 . In FIG. 2 , t0 is a point in time whenthe output voltage of the driver 3 is changed from the positive biasvoltage into 0 V (or a negative bias voltage), for example, in theswitch device 100, t1 is a point in time when the discharge current Idisstarts to flow, t2 is a point in time when the source current Is of theswitching element 1 that has been increasing starts to decrease, and t3is a point in time when the source current Is becomes equal to zero.

In the switch device 100, right after the switching element 1 hasstarted to turn OFF, the potential at the source S1 and theelectromotive force of the inductor L1 are 0 V and the referencepotential Vstd is approximately equal to the source potential, i.e., 0V.

In the switch device 100, until the point in time t2 when the sourcecurrent Is that has been increasing starts to decrease, the gate G1 ofthe switching element 1 is discharged via the drive circuit 2, andtherefore, the discharge current Idis flows from the gate G1. At thistime, in the switch device 100, the gate-source voltage Vgs of theswitching element 1 decreases steeply and then becomes substantiallyconstant.

In the switch device 100, once the source current Is has started todecrease at the point in time t2, the current value of the dischargecurrent Idis decreases and the rate of decrease in the gate potentialslows down, thus enabling decreasing the variation rate (dIs/dt) of thesource current Is and thereby reducing the surge voltage applied to theswitching element 1.

In the switch device 100, the induced electromotive force VL generatedin the inductor L1 as the source current Is decreases causes an increasein the reference potential Vstd via the capacitor C1. More specifically,in the control circuit 10, the induced electromotive force generated inthe inductor L1 as the source current Is decreases makes the potentialat the second terminal of the inductor L1 higher than the potential atthe first terminal thereof, thus making the potential at the node N2higher than the potential at the source S1. As a result, an electriccurrent flows through a closed-loop circuit including the inductor L1,the capacitor C1, and the resistor R1. That is to say, in the controlcircuit 10, the electric current flows through the capacitor C1 as thecircuit element 5 (i.e., the capacitor C1 as a circuit element 5 turnselectrically conductive). Consequently, in the switch device 100, thereference potential Vstd becomes higher than the potential at the sourceS1 to decrease the potential difference between the gate potential andthe reference potential Vstd. Thus, the current value of the dischargecurrent Idis flowing from the gate G1 of the switching element 1decreases to slow down the rate of decrease in the source current Is. Asa result, the electric current may be cut off gently.

The control circuit 10 discharges the gate G1 at a higher rate in theperiod from the point in time t1 through the point in time t2(hereinafter referred to as a “first period”) than in the period fromthe point in time t2 through the point in time t3 (hereinafter referredto as a “second period”). In other words, the control circuit 10discharges the gate G1 at a lower rate in the second period than in thefirst period. This allows the switch device 100 to turn OFF in a shortertime by shortening the period between the point in time t1 and the pointin time t2 and to decrease the absolute value of the current variationrate of the source current Is between the points in time t2 and t3, thusenabling reducing the surge voltage applied to the switching element 1.

As can be seen from the foregoing description, in the control circuit10, as the source current Is flowing through the source S1 when theswitching element 1 turns OFF decreases, electromotive force isgenerated in the inductor L1 and an electric current flows in accordancewith the electromotive force through the circuit element 5 (capacitorC1) and the resistor R1. This causes an increase in potential at thereference potential point P0 included in the path between the node N3,to which the circuit element 5 is connected, and the gate G1 of theswitching element 1. Consequently, in the control circuit 10, thepotential difference between the potential at the gate G1 of theswitching element 1 and the potential at the reference potential pointP0 determines the magnitude of the discharge current Idis flowing fromthe gate G1.

The control circuit 10 may control the switching element 1 using theinductor L1, the resistor R1, and the capacitor C1. While a current isflowing through the closed-loop circuit including the inductor L1, thecapacitor C1, and the resistor R1 with the electromotive force of theinductor L1, the reference potential Vstd at the reference potentialpoint P0 becomes higher than the source potential, thus decreasing thepotential difference between the potential at the gate G1 and thereference potential Vstd and thereby decreasing the discharge currentIdis flowing from the gate G1. This allows the control circuit 10 tochange the current variation rate of the source current Is (in otherwords, the rate at which the source current Is is cut off) during thesecond period by changing at least one of the capacitance of thecapacitor C1, the resistance value of the resistor R1, or the inductanceof the inductor L1. For example, if the capacitance of the capacitor C1of the control circuit 10 is changed, then the characteristics are thesame during the first period but the current variation rates during thesecond period are different from each other. FIG. 3 shows the waveformsof the source current Is that were obtained when the capacitance of thecapacitor C1 was changed into various values in the control circuit 10.In FIG. 3 , the characteristics during the second period are indicatedby four different types of curves. In the example shown in FIG. 3 , thecapacitance of the capacitor C1 increases in the order of thecharacteristics B1, B2, B3, and B4. It can be seen from FIG. 3 that asthe capacitance of the capacitor C1 increases, the cutoff rate of thesource current Is slows down. In the control circuit 10, even if not thecapacitance of the capacitor C1 but the resistance value of the resistorR1 or the inductance of the inductor L1 is increased, the cutoff rate ofthe source current Is also slows down. Specifically, in the controlcircuit 10, as the resistance value of the resistor R1 is increased, theswitching rate decreases when the switching element 1 turns OFF. That isto say, in the control circuit 10, the absolute value of the currentvariation rate (di/dt) of the source current Is flowing through theswitching element 1 decreases. Also, in the control circuit 10, as theinductance of the inductor L1 is increased, the size of the inductor L1increases and the size of the control circuit 10 also increases. Thus,to reduce the chances of causing a decrease in the switching rate whenthe switching element 1 turns ON and to reduce an increase in the sizeof the control circuit 10, it is advantageous for the control circuit 10to determine the switching rate when the switching element 1 turns OFFby the capacitance of the capacitor C1. Note that in the control circuit10, the inductor L1 may have an inductance of 50 nH, the resistor R1 mayhave a resistance value of 1 Ω, and the capacitor C1 may have acapacitance of 100 nF, for example. However, these numerical values areonly examples and should not be construed as limiting. Furthermore, thegate resistor included in the drive circuit 2 may have a resistancevalue of 50 Ω, which is only an example and should not be construed aslimiting, either.

In a comparative example in which the control circuit 10 includes nocircuit element 5, not only the discharge current Idis that flows duringthe period from the point in time t2 through the point in time t3 butalso the absolute value of the current variation rate during the periodfrom the point in time t2 through the point in time t3 may be increasedwhen the switching element 1 turns OFF, compared to the control circuit10 including the circuit element 5. Thus, the comparative exampleenables shortening the switching time and cutting down the switchingloss. According to the comparative example, however, a surge voltage maybe generated in the switching element 1 to cause a failure in theswitching element 1. In addition, according to the comparative example,decreasing the discharge current Idis during the period from the pointin time t2 through the point in time t3 may reduce the chances ofgenerating the surge voltage but may also extend the switching time andcause an increase in switching loss. In contrast, in the switch device100 including the control circuit 10 according to this embodiment, thedischarge current Idis flows from the gate G1 of the switching element 1in accordance with the potential difference between the gate potentialand the reference potential Vstd. In the period from the point in timet2 through the point in time t3, the electric current flowing throughthe closed-loop circuit including the inductor L1, the circuit element5, and the resistor R1 decreases the potential difference between thegate potential and the reference potential Vstd, thus decreasing thedischarge current Idis and the absolute value of the current variationrate of the source current Is. Thus, the control circuit 10 according tothis embodiment allows different amounts of discharge current Idis toflow in the first period from the point in time t1 through the point intime t2 and in the second period from the point in time t2 through thepoint in time t3 when the switching element 1 turns OFF, thus enablingcutting down the switching loss by increasing the discharge current toflow in the first period and reducing the surge voltage by decreasingthe discharge current to flow in the second period. As used herein, theswitching loss involved when the switching element 1 turns OFF refers tothe power loss caused by the switching element 1 when the switchingelement 1 implemented as, for example, a semiconductor switch turns OFF.

In addition, in the switch device 100, the amount of the electriccurrent flowing through the resistor R1 of the control circuit 10increases when the switching element 1 turns ON. This causes thereference potential Vstd to increase and also causes the potential atthe gate G1 to rise more gently.

(4) Advantages

A control circuit 10 according to the first embodiment controls aswitching element including a gate G1 and a source S1 corresponding tothe gate G1. The control circuit 10 includes an inductor L1, a capacitorC1 as a circuit element 5, and a resistor R1. The inductor L1 isconnected between the gate G1 and the source S1 of the switching element1. The circuit element 5 is connected in series to the inductor L1between the gate G1 and the source S1. The circuit element 5 allows anelectric current to flow therethrough in response to generation ofelectromotive force in the inductor L1.

The control circuit 10 according to the first embodiment may reduce asurge voltage applied to the switching element 1 while cutting down theswitching loss involved when the switching element 1 turns OFF.

In addition, the switch device 100 according to the first embodimentincludes the switching element 1 and the control circuit 10, andtherefore, may also reduce a surge voltage applied to the switchingelement 1 while cutting down the switching loss involved when theswitching element 1 turns OFF.

Variations of First Embodiment

Next, variations of the control circuit 10 and switch device 100according to the first embodiment will be enumerated one after another.Note that the variations to be described below may be adopted asappropriate in combination with the control circuit 10 and switch device100 according to the first embodiment. In the following description, anyconstituent element having the same function as a counterpart of thecontrol circuit 10 and switch device 100 according to the firstembodiment described above will be designated by the same referencenumeral as that counterpart's, and description thereof will be omittedherein.

First Variation of First Embodiment

Next, a control circuit 10 a according to a first variation of the firstembodiment and a switch device 100 a including the control circuit 10 awill be described with reference to FIG. 4 .

The control circuit 10 a includes a negative power supply V1, which is adifference from the control circuit 10 according to the firstembodiment. In this variation, the negative power supply V1 is connectedbetween the node N3 and the low-potential output terminal of the driver3 (hereinafter referred to as a “negative-side terminal”). In the switchdevice 100 a, the negative-side terminal of the negative power supply V1is connected to the negative-side terminal of the driver 3. In the otherrespects, the control circuit 10 a has the same configuration as thecontrol circuit 10 (see FIG. 1 ) according to the first embodiment.

Second Variation of First Embodiment

Next, a control circuit 10 b according to a second variation of thefirst embodiment and a switch device 100 b including the control circuit10 b will be described with reference to FIG. 5 .

In the control circuit 10 b according to the second variation, thecircuit element 5 is a diode Di1, which is a difference from the controlcircuit 10 according to the first embodiment. The diode Di1 has an anodeand a cathode. The anode of the diode Di1 is connected to the node N2.The cathode of the diode Di1 is connected to the node N3. That is tosay, in the control circuit 10 b, the resistor R1 is connected betweenthe first terminal of the inductor L1 and the cathode of the diode Di1.

The circuit operation of this control circuit 10 b in which thecapacitor C1 of the control circuit 10 is replaced with the diode Di1 isthe same as the circuit operation of the control circuit 10. In thecontrol circuit 10 b, the electromotive force (counter electromotiveforce) generated by the inductor L1 is consumed by the diode Di1 and theresistor R1 in the closed-loop circuit including the inductor L1, thediode Di1, and the resistor R1. In this control circuit 10 b, the cutoffrate of the source current Is when the switching element 1 turns OFF maybe slowed down by increasing the inductance of the inductor L1, forexample. FIG. 6 shows the waveforms of the source current Is that wereobtained when the inductance of the inductor L1 was changed into variousvalues in the control circuit 10 b. In FIG. 6 , the characteristicsduring the second period in which the source current Is decreases areindicated by four different types of curves. In the example shown inFIG. 6 , the inductance of the inductor L1 increases in the order of thecharacteristics B5, B6, B7, and B8. It can be seen from FIG. 6 that asthe inductance of the inductor L1 increases, the cutoff rate of thesource current Is slows down.

Also, in this control circuit 10 b, as the resistance value of theresistor R1 is increased, the time constant of the series circuit of theresistor R1 and the inductor L1 decreases. Thus, increasing theresistance value of the resistor R1 is one of means for increasing thecutoff rate of the source current Is when the switching element 1 turnsOFF. Meanwhile, in this control circuit 10 b, increasing the resistancevalue of the resistor R1 means that the resistance value of the resistorR1, located on the path, through which the discharge current Idis fromthe gate G1 of the switching element 1 flows via the drive circuit 2,increases. Thus, increasing the resistance value of the resistor R1 isalso one of means for decreasing the current cutoff rate of the sourcecurrent Is when the switching element 1 turns OFF. In this controlcircuit 10, the relation between the resistance value of the resistor R1and the cutoff rate of the source current Is depends on the combinationof other circuit parameters. Thus, it is easier for the control circuit10 b to adjust the cutoff rate of the source current Is with theinductance of the inductor L1 rather than adjusting the cutoff rate ofthe source current Is with the resistance value of the resistor R1.

Also, in the switch device 100 including the control circuit 10according to the first embodiment, after the source current Is of theswitching element 1 has been cut off, an electric current may flow, as aflow of the electric charge stored in, and drained from, the capacitorC1 to make the gate-source voltage Vgs of the switching element 1negative in some cases (i.e., make the potential at the source S1 higherthan the potential at the gate G1). On the other hand, the switch device100 b including the control circuit 10 b according to this variationincludes, as the circuit element 5, the diode Di1 instead of thecapacitor C1, and therefore, the discharge current flowing from thecircuit element 5 decreases after the source current Is has been cutoff. Thus, even if the gate-source voltage Vgs of the switching element1 becomes negative, the absolute value thereof may still be decreased.

Optionally, the control circuit 10 according to the first embodiment maybe combined with the control circuit 10 b according to this variation.Specifically, a control circuit that adopts such a combination has aconfiguration in which the capacitor C1 is connected in series to thediode Di1 of the control circuit 10 b according to this variation andthe resistor R1 is connected in parallel to the inductor L1, the diodeDi1, and the capacitor C1, and therefore, has two circuit elements 5which are connected in series to the inductor L1. If one of the twocircuit elements 5 is hereinafter referred to as a “first circuitelement” and the other circuit element 5 as a “second circuit element,”the first circuit element is the diode Di1 and the second circuitelement is the capacitor C1, for example.

Third Variation of First Embodiment

Next, a control circuit 10 c according to a third variation of the firstembodiment and a switch device 100 c including the control circuit 10 cwill be described with reference to FIG. 7 .

In the control circuit 10 c according to this variation, a protectivediode Di2 is further provided for the control circuit 10 according tothe first embodiment, which is a difference from the control circuit 10according to the first embodiment. The protective diode Di2 includes ananode and a cathode. The protective diode Di2 may be a Schottky diode,for example, but may also be a different type of diode from the Schottkydiode.

The protective diode Di2 is connected between the reference potentialpoint P0 and the gate G1 to form a different path from the path thatconnects the node N3 and the gate G1 together. Specifically, in thiscontrol circuit 10 c, the anode of the protective diode Di2 is connectedto a node N7 located on the path between the negative-side terminal ofthe driver and the node N3. The protective diode Di2 is connected to apoint of connection between the resistor R1 and the circuit element 5.Thus, in the switch device 100 c including this control circuit 10 c,the anode of the protective diode Di2 is connected to the negative-sideterminal of the driver 3, and therefore, comes to have substantially thesame potential as the potential at the reference potential point P0. Onthe other hand, the cathode of the protective diode Di2 is connected toa node N8 located on the path between the drive circuit 2 and the gateG1 of the switching element 1, and therefore, comes to havesubstantially the same potential as the potential at the gate G1 of theswitching element 1.

In the switch device 100 c including this control circuit 10 c, afterthe source current Is of the switching element 1 has been cut off, theelectric charge stored in the capacitor C1 flows as an electric currentI5 through, and is consumed by, a closed-loop circuit including thecapacitor C1, the inductor L1, and the resistor R1. When the sourcecurrent Is has been cut off completely, the potential at the gate G1 ofthe switching element 1 is approximately equal to the potential at thereference potential point P0. Thus, in the switch device 100 c, as theelectric current I5 flows, the gate potential becomes lower than thesource potential to make the gate-source voltage Vgs negative. In theswitch device 100 c, when the gate-source voltage Vgs of the switchingelement 1 becomes negative, the protective diode Di2 operates to makethe gate-source voltage Vgs constant. As a result, in the switch device100 c, the gate-source voltage Vgs becomes approximately equal to theconduction voltage of the protective diode Di2. Thus, in the switchdevice 100 c, the switching element 1 is protected.

Optionally, the control circuit 10 c may further include anotherresistor which is connected in series to the protective diode Di2between the nodes N7 and N8 to prevent the protective diode Di2 fromcausing dielectric breakdown, for example.

Optionally, the control circuit 10 c may further include a negativepower supply, of which the positive-side terminal is connected to thenode N7, between the node N7 and the negative-side terminal of thedriver 3. In that case, the protective diode Di2 is preferablyimplemented as a series circuit of a plurality of diodes to prevent theprotective diode Di2 from being kept electrically conductive with thevoltage of the negative power supply. This increases the forward voltageof the protective diode Di2, thus enabling preventing the protectivediode Di2 from being kept electrically conductive with the voltage ofthe negative power supply.

Fourth Variation of First Embodiment

Next, a control circuit 10 d according to a fourth variation of thefirst embodiment and a switch device 100 d including the control circuit10 d will be described with reference to FIG. 8 .

The control circuit 10 d according to the fourth variation includes aprotective diode Di3, which is connected between the gate G1 and sourceS1 of the switching element 1, which is a difference from the controlcircuit 10 according to the first embodiment. The protective diode Di3includes an anode and a cathode. The protective diode Di3 may be aSchottky diode, for example, but may also be a different type of diodefrom the Schottky diode. In the protective diode Di3, the anode of theprotective diode Di3 is connected to the source S1 of the switchingelement 1 and the cathode of the protective diode Di3 is connected tothe gate G1 of the switching element 1. In the switch device 100 dincluding this control circuit 10 d, the anode of the protective diodeDi3 is connected to a node N9. The node N9 is located on the pathbetween the source S1 of the switching element 1 and a node N1 betweenthe inductor L1 and the resistor R1. The cathode of the protective diodeDi3 is connected to a node N10 located on the path between the gate G1of the switching element 1 and the drive circuit 2.

In the control circuit 10 d according to the fourth variation, theprotective diode Di3 is connected between the gate G1 and source S1 ofthe switching element 1. This enables keeping the gate-source voltageVgs constant (i.e., clamping the gate-source voltage Vgs) with theforward voltage of the protective diode Di3 when the electric chargestored in the capacitor C1 flows as an electric current I5 (see FIG. 7 )through the closed-loop circuit including the capacitor C1, the inductorL1, and the resistor R1 after the source current Is of the switchingelement 1 has been cut off. This allows the control circuit 10 d toreduce the chances of the potential at the source S1 of the switchingelement 1 increasing too much with respect to the potential at the gateG1, thus enabling protecting the switching element 1.

Other Variations of First Embodiment

In the control circuit 10 according to the first embodiment, theresistor R1 is an electronic component (resistor). However, this is onlyan example and should not be construed as limiting. Alternatively, theresistor R1 may also be implemented as, for example, a cable havingelectrical conductivity (i.e., an electric wire cable). The resistancevalue of the resistor R1 may be less than 1 Ω and may be closer to 0 Ωthan to 1 Ω.

In the control circuit 10 according to the first embodiment, theinductor L1 is an electronic component (e.g., a surface-mountedinductor). However, this configuration is only an example and should notbe construed as limiting. Alternatively, the inductor L1 may also beimplemented as, for example, a cable having electrical conductivity(i.e., an electric wire cable). That is to say, the inductor L1 may alsobe configured to use parasitic inductance.

Second Embodiment

Next, a switch device 100 e according to a second embodiment will bedescribed with reference to FIG. 9 .

If the configuration of the switch device 100 according to the firstembodiment is called a “basic circuit,” the switch device 100 eaccording to the second embodiment has two basic circuits and includes abidirectional switch formed by connecting together the respectiveswitching elements 1 of the two basic circuits, which is a differencefrom the first embodiment. In the following description, any constituentelement of the switch device 100 e according to this second embodiment,having the same function as a counterpart of the switch device 100according to the first embodiment described above, will be designated bythe same reference numeral as that counterpart's, and descriptionthereof will be omitted herein.

A bidirectional switch is an important device for replacing a powerconverter circuit, which has been implemented as an inverter circuit anda converter circuit, with a power converter circuit of a matrixconverter type. The power converter circuit of the matrix converter typemay convert, for example, AC power into AC power with an arbitraryfrequency by turning ON and OFF, at high speeds, bidirectional switchesthat are arranged in a matrix pattern.

The switch device 100 e includes two switching elements 1 and twocontrol circuits 10, which is a difference from the switch device 100according to the first embodiment. Also, in the switch device 100 e, thetwo switching elements 1 are connected in series and the two controlcircuits 10 are associated one to one with the two switching elements 1.

Each of the two switching elements 1 includes a source S1, a gate G1,and a drain D1. In this switch device 100 e, the respective drains D1 ofthe two switching elements 1 are connected to each other. In this switchdevice 100 e, a bidirectional switch is formed by these two switchingelements 1. In the following description, out of the two switchingelements 1, the lower switching element 1 in FIG. 9 will be hereinafterreferred to as a “first switching element 1A” and the upper switchingelement 1 in FIG. 9 will be hereinafter referred to as a “secondswitching element 1B” for the sake of convenience of description. Also,in the following description, the source S1, gate G1, and drain D1 ofthe first switching element 1A will be hereinafter referred to as a“first source S11,” a “first gate G11,” and a “first drain D11,”respectively, and the source S1, gate G1, and drain D1 of the secondswitching element 1B will be hereinafter referred to as a “second sourceS12,” a “second gate G12,” and a “second drain D12,” respectively.Furthermore, in the following description, out of the two controlcircuits 10, the control circuit 10 associated with the first switchingelement 1A will be hereinafter referred to as a “first control circuit10 e 1” and the control circuit 10 associated with the second switchingelement 1B will be hereinafter referred to as a “second control circuit10 e 2.” Furthermore, in the following description, the inductor L1 ofthe first control circuit 10 e 1 will be hereinafter referred to as a“first inductor L11” and the inductor L1 of the second control circuit10 e 2 will be hereinafter referred to as a “second inductor L12.”Furthermore, in the following description, the driver 3 associated withthe first switching element 1A will be hereinafter referred to as a“first driver 3A” and the driver 3 associated with the second switchingelement 1B will be hereinafter referred to as a “second driver 3B.”Furthermore, in the following description, the drive circuit 2associated with the first switching element 1A will be hereinafterreferred to as a “first drive circuit 2A” and the drive circuit 2associated with the second switching element 1B will be hereinafterreferred to as a “second drive circuit 2B.” Furthermore, the potentialat the reference potential point P0 between the node N3 of the firstcontrol circuit 10 e 1 and the low-potential output terminal of thefirst driver 3A will be hereinafter referred to as a “first referencepotential Vstd1” and the potential at the reference potential point P0between the node N3 of the second control circuit 10 e 2 and thelow-potential output terminal of the second driver 3B will behereinafter referred to as a “second reference potential Vstd2.”Furthermore, in the bidirectional switch including the two switchingelements 1, the electric current flowing from the second source S12toward the first source S11 will be hereinafter referred to as a “sourcecurrent Is2 s 1” and the electric current flowing from the first sourceS11 toward the second source S12 will be hereinafter referred to as a“source current Is1 s 2.” In the switch device 100 e, a load circuitincluding a load and a power supply is connected between a firstterminal at one end of a series circuit including the first inductorL11, the first switching element 1A, the second switching element 1B,and the second inductor L12 and a second terminal at the other endthereof.

Next, the operation of the switch device 100 e will be described at thetime of turn OFF when the bidirectional switch is turned OFF from astate where the source current Is2 s 1 is flowing through thebidirectional switch including the two switching elements 1 (i.e., whenthe two switching elements 1 are in ON state and the bidirectionalswitch is in ON state). As used herein, “to tun OFF the bidirectionalswitch” means turning OFF both the first switching element 1A and thesecond switching element 1B.

In the switch device 100 e, when the source current Is1 s 2 that hasbeen increasing starts to decrease after the bidirectional switch hasstarted to be turned OFF, counter electromotive force (inducedelectromotive force) is generated in each of the first inductor L11 andthe second inductor L12. In the switch device 100 e, when the counterelectromotive force is generated in the first inductor L11, the firstreference potential Vstd1 becomes higher than the potential at the firstsource S11. As a result, in the switch device 100 e, the potentialdifference between the potential at the first gate G11 of the firstswitching element 1A and the first reference potential Vstd1 decreases,and therefore, the discharge current flowing from the first gate G11 ofthe first switching element 1A also decreases, thus causing the cutoffrate of the source current Is2 s 1 to slow down.

On the other hand, in the switch device 100 e, when the counterelectromotive force is generated in the second inductor L12, the secondreference potential Vstd2 becomes lower than the source potential of thesecond switching element 1B. As a result, in the switch device 100 e,the potential difference between the second gate G12 of the secondswitching element 1B and the second reference potential Vstd2 increases,thus turning the second switching element 1B OFF before the firstswitching element 1A turns OFF. From the viewpoint of cutting off thesource current Is2 s 1 flowing through the bidirectional switch, thesource current Is2 s 1 flows through the second switching element 1B, nomatter whether the second switching element 1B is ON or OFF. Thus, theturn-off rate of the second switching element 1B does not affect cutoffof the principal current (source current Is2 s 1) of the bidirectionalswitch.

In the first control circuit 10 e 1 associated with the first switchingelement 1A, after the source current Is2 s 1 of the bidirectional switchhas been cut off, an electric current I7 flows, as a flow of theelectric charge that has been stored in, and drained from, the capacitorC1, through a closed-loop circuit including the capacitor C1, theresistor R1, and the first inductor L11. On the other hand, in thesecond control circuit 10 e 2 associated with the second switchingelement 1B, after the source current Is2 s 1 has been cut off, anelectric current I8 flows, as a flow of the electric charge that hasbeen stored in, and drained from, the capacitor C1, through aclosed-loop circuit including the capacitor C1, the resistor R1, and thesecond inductor L12.

Next, the operation of the switch device 100 e will be described at thetime of turn OFF when the bidirectional switch is turned OFF from astate where the source current Is1 s 2 is flowing through thebidirectional switch including the two switching elements 1 (i.e., whenthe two switching elements 1 are in ON state and the bidirectionalswitch is in ON state).

In the switch device 100 e, when the source current Is2 s 1 that hasbeen increasing starts to decrease after the bidirectional switch hasstarted to be turned OFF, counter electromotive force (inducedelectromotive force) is generated in each of the first inductor L11 andthe second inductor L12. In the switch device 100 e, when the counterelectromotive force is generated in the first inductor L11, the firstreference potential Vstd1 becomes lower than the source potential of the1 switching element 1A. As a result, the potential difference betweenthe gate potential of the first switching element 1A and the firstreference potential increases, and therefore, the first switchingelement 1A turns OFF before the second switching element 1B turns OFF.

On the other hand, in the switch device 100 e, when the counterelectromotive force is generated in the second inductor L12, the secondreference potential Vstd2 becomes higher than the source potential ofthe second switching element 1B. As a result, in the switch device 100e, the potential difference between the second gate G12 of the secondswitching element 1B and the second reference potential Vstd2 decreases,and therefore, the discharge current flowing from the second gate G2 ofthe second switching element 1B decreases, thus causing the cutoff rateof the source current Is1 s 2 to slow down.

The switch device 100 e according to the second embodiment includes twoswitching elements 1 and two control circuits 10 associated one to onewith the two switching elements 1. This enables reducing a surge voltageapplied to each of the two switching elements 1 while cutting down theswitching loss involved when each of the switching elements 1 turns OFF.

In addition, the switch device 100 e according to the second embodimentalso enables reducing a surge voltage applied to the bidirectionalswitch while cutting down the switching loss involved when thebidirectional switch turns OFF.

Variations of Second Embodiment

Next, variations of the switch device 100 e according to the secondembodiment will be enumerated one after another. Note that thevariations to be described below may be adopted as appropriate incombination with the first and second embodiments described above.

First Variation of Second Embodiment

Next, a switch device 100 f according to a first variation of the secondembodiment will be described with reference to FIG. 10 .

The switch device 100 e according to the second embodiment includes thebidirectional switch formed by connecting together the respective drainsD1 of the two switching elements 1 as described above. On the otherhand, the switch device 100 f according to the first variation of thesecond embodiment includes a single switching element 1 f instead of thetwo switching elements 1, which is difference from the switch device 100e according to the second embodiment. The switching element 1 f is adual-gate bidirectional switch including two gates G1 and two sourcesS1.

In the switching element 1 f, the two gates G1 and the two sources S1correspond one to one to each other. In the following description, inthe switching element 1 f, one of the two gates G1 will be hereinafterreferred to as a “first gate G11” and the other gate G1 as a “secondgate G12” for the sake of convenience of description. In the same way,out of the two sources S1, the source S1 corresponding to the first gateG11 will be hereinafter referred to as a “first source S11” and thesource S1 corresponding to the second gate G12 will be hereinafterreferred to as a “second source S12.”

In the following description, the switching element 1 f will bedescribed briefly first, and then the switch device 100 f will bedescribed.

The switching element 1 f is a type of GaN-based GIT. The switchingelement 1 f includes, for example, a substrate, a buffer layer, a firstnitride semiconductor layer, a second nitride semiconductor layer, afirst source electrode, a first gate electrode, a second gate electrode,a second source electrode, a first p-type layer, and a second p-typelayer. The buffer layer is formed on the substrate. The first nitridesemiconductor layer is formed on the buffer layer. The second nitridesemiconductor layer is formed on the first nitride semiconductor layer.The first source electrode, the first gate electrode, the second gateelectrode, and the second source electrode are formed on the secondnitride semiconductor layer. The first p-type layer is interposedbetween the first gate electrode and the second nitride semiconductorlayer. The second p-type layer is interposed between the second gateelectrode and the second nitride semiconductor layer. In the switchingelement 1 f, the first source S11 includes the first source electrode.The first gate G11 includes the first gate electrode and the firstp-type layer. The second gate G12 includes the second gate electrode andthe second p-type layer. The second source S12 includes the secondsource electrode. The substrate is a silicon substrate, for example. Thebuffer layer is an undoped GaN layer, for example. The first nitridesemiconductor layer is, for example, an undoped GaN layer. The secondnitride semiconductor layer is, for example, an undoped AlGaN layer.Each of the first p-type layer and the second p-type layer is, forexample, a p-type AlGaN layer. Each of the buffer layer, the firstnitride semiconductor layer, and the second nitride semiconductor layermay include impurities such as Mg, H, Si, C, and O to be inevitablycontained during their growing process by metal-organic vapor phaseepitaxy (MOVPE), for example.

In the switching element 1 f, the second nitride semiconductor layerforms, along with the first nitride semiconductor layer, aheterojunction portion. In the first nitride semiconductor layer, atwo-dimensional electron gas has been generated in the vicinity of theheterojunction portion. A region including the two-dimensional electrongas (hereinafter referred to as a “two-dimensional electron gas layer”)may also serve as an n-channel layer (electron conduction layer).

In the following description, out of the two control circuits 10, thecontrol circuit 10 connected between the first gate G11 and the secondsource S11 of the switching element 1 f will be hereinafter referred toas a “first control circuit 10 f 1” and the control circuit 10 connectedbetween the second gate G12 and the second source S12 of the switchingelement 1 f will be hereinafter referred to as a “second control circuit10 f 2.” Furthermore, in the following description, the inductor L1 ofthe first control circuit 10 f 1 will be hereinafter referred to as a“first inductor L11” and the inductor L1 of the second control circuit10 f 2 will be hereinafter referred to as a “second inductor L12.”Furthermore, in the following description, the driver 3 associated withthe first gate G11 of the switching element 1 f will be hereinafterreferred to as a “first driver 3A” and the driver 3 associated with thesecond gate G12 of the switching element 1 f will be hereinafterreferred to as a “second driver 3B.” Furthermore, in the followingdescription, the drive circuit 2 associated with the first gate G11 ofthe switching element 1 f will be hereinafter referred to as a “firstdrive circuit 2A” and the drive circuit 2 associated with the secondgate G2 of the switching element 1 f will be hereinafter referred to asa “second drive circuit 2B.” Furthermore, the potential at the referencepotential point P0 between the node N3 of the first control circuit 10 f1 and the low-potential output terminal of the first driver 3A will behereinafter referred to as a “first reference potential Vstd1” and thepotential at the reference potential point P0 between the node N3 of thesecond control circuit 10 f 2 and the low-potential output terminal ofthe second driver 3B will be hereinafter referred to as a “secondreference potential Vstd2.” Furthermore, in the switching element 1 f,the electric current flowing from the second source S12 toward the firstsource S11 will be hereinafter referred to as a “source current Is2 s 1”and the electric current flowing from the first source S11 toward thesecond source S12 will be hereinafter referred to as a “source currentIs1 s 2.”

Also, in the following description, a state where a voltage equal to orhigher than a first threshold voltage (of 1.3 V, for example) is notapplied between the first gate G11 and the first source S11 with thefirst gate G11 having the higher potential will be hereinafter referredto as a “state where the first gate G11 is OFF.” Also, a state where avoltage equal to or higher than the first threshold voltage is appliedbetween the first gate G11 and the first source S11 with the first gateG11 having the higher potential will be hereinafter referred to as a“state where the first gate G11 is ON.” Furthermore, a state where avoltage equal to or higher than a second threshold voltage (of 1.3 V,for example) is not applied between the second gate G12 and the secondsource S12 with the second gate G12 having the higher potential will behereinafter referred to as a “state where the second gate G12 is OFF.”Also, a state where a voltage equal to or higher than the secondthreshold voltage is applied between the second gate G12 and the secondsource S12 with the second gate G12 having the higher potential will behereinafter referred to as a “state where the second gate G12 is ON.”

This switching element 1 f includes the first p-type layer and thesecond p-type layer, thus implementing a normally OFF transistor.

The switching element 1 f may be switched from one of a bidirectionallyON state, a bidirectionally OFF state, a first diode state, or a seconddiode state to another depending on the combination of a first gatevoltage applied to the first gate G11 and a second gate voltage appliedto the second gate G12. The first gate voltage is a voltage appliedbetween the first gate G11 and the first source S11. The second gatevoltage is a voltage applied between the second gate G12 and the secondsource S12. The bidirectionally ON state is a state where an electriccurrent is allowed to pass bidirectionally (i.e., in a first directionand a second direction opposite from the first direction). Thebidirectionally OFF state is a state where an electric current isblocked bidirectionally. The first diode state is a state where anelectric current is allowed to pass in the first direction. The seconddiode state is a state where an electric current is allowed to pass inthe second direction. The electric current in the first direction is thesource current Is1 s 2. The electric current in the second direction isthe source current Is2 s 1.

In a state where the first gate G11 is ON and the second gate G12 is ON,the switching element 1 f turns into the bidirectionally ON state. In astate where the first gate G11 is OFF and the second gate G12 is OFF,the switching element 1 f turns into the bidirectionally OFF state. In astate where the first gate G11 is OFF and the second gate G12 is ON, theswitching element 1 f turns into the first diode state. In a state wherethe first gate G11 is ON and the second gate G12 is OFF, the switchingelement 1 f turns into the second diode state.

In the switch device 100 f, a load circuit including a load and a powersupply is connected between a first terminal at one end of a seriescircuit including the first inductor L11, the switching element 1 f, andthe second inductor L12 and a second terminal at the other end thereof.Next, the operation of the switch device 100 f will be described at thetime of turn OFF when the switching element 1 f is turned OFF from astate where the switching element 1 f is in ON state and the sourcecurrent Is2 s 1 is flowing. The first control circuit 10 f 1 and thesecond control circuit 10 f 2 operate in the same way as the firstcontrol circuit 10 e 1 and the second control circuit 10 e 2,respectively.

In the switch device 100 f, when the source current Is1 s 2 that hasbeen increasing starts to decrease after the switching element 1 f hasstarted to be turned OFF, counter electromotive force (inducedelectromotive force) is generated in each of the first inductor L11 andthe second inductor L12.

In the switch device 100 f, when the counter electromotive force isgenerated in the first inductor L11, the first reference potential Vstd1becomes higher than the potential at the first source 11. As a result,in the switch device 100 f, the potential difference between thepotential at the first gate G11 of the switching element 1 f and thefirst reference potential Vstd1 decreases, and therefore, the dischargecurrent flowing from the first gate G11 also decreases, thus causing thecutoff rate of the source current Is2 s 1 to slow down.

On the other hand, in the switch device 100 f, when the counterelectromotive force is generated in the second inductor L12, the secondreference potential Vstd2 becomes lower than the potential at the secondsource S12. As a result, in the switch device 100 f, the potentialdifference between the potential at the second gate G12 and thereference potential Vstd2 increases, thus turning the second gate G12OFF.

In the switch device 100 f, even if the second gate G12 has turned OFF,the source current Is2 s 1 continues to flow as long as the first gateG11 is in ON state. Once the first gate G11 has turned OFF, the sourcecurrent Is2 s 1 is cut off.

Next, the operation of the switch device 100 f will be described at thetime of turn OFF when the switching element 1 f is turned OFF from astate where the source current Is1 s 2 is flowing through the switchingelement 1 f.

In the switch device 100 f, when the source current Is1 s 2 that hasbeen increasing starts to decrease after the bidirectional switch hasstarted to be turned OFF, counter electromotive force (inducedelectromotive force) is generated in each of the first inductor L11 andthe second inductor L12.

In the switch device 100 f, when the counter electromotive force isgenerated in the second inductor L12, the second reference potentialVstd2 becomes higher than the potential at the second source S12. As aresult, in the switch device 100 f, the potential difference between thepotential at the second gate G12 and the second reference potentialVstd2 decreases, and therefore, the discharge current flowing from thesecond gate G12 also decreases, thus causing the cutoff rate of thesource current Is1 s 2 to slow down.

On the other hand, in the switch device 100 f, when the counterelectromotive force is generated in the first inductor L11, the firstreference potential Vstd1 becomes lower than the potential at the firstsource S11. As a result, in the switch device 100 f, the potentialdifference between the potential at the first gate G11 and the firstreference potential Vstd1 increases, thus turning the first gate G11OFF.

In the switch device 100 f, even if the first gate G11 has turned OFF,the source current Is1 s 2 continues to flow as long as the second gateG12 is in ON state. Once the second gate G12 has turned OFF, the sourcecurrent Is1 s 2 is cut off.

As can be seen from the foregoing description, the switch device 100 fmay slow down the cutoff rate with respect to each of the bidirectionalsource currents Is2 s 1, Is1 s 2, thus reducing the surge voltageapplied to the switching element 1 f.

Thus, the switch device 100 f according to the first variation of thesecond embodiment may reduce a surge voltage applied to the switchingelement 1 f while cutting down the switching loss involved when theswitching element 1 f turns OFF.

Second Variation of Second Embodiment

Next, a switch device 100 g according to a second variation of thesecond embodiment will be described with reference to FIG. 11 .

The switch device 100 e according to the second embodiment includes thebidirectional switch formed by connecting together the respective drainsD1 of the two switching elements 1. On the other hand, in the switchdevice 100 g according to the second variation, the respective sourcesS1 of the two switching elements 1 are connected together, which is adifference from the switch device 100 e according to the secondembodiment.

In the following description, out of the two switching elements 1, theupper switching element 1 in FIG. 11 will be hereinafter referred to asa “first switching element 1A” and the lower switching element 1 in FIG.11 will be hereinafter referred to as a “second switching element 1B”for the sake of convenience of description. Also, in the followingdescription, the source S1, gate G1, and drain D1 of the first switchingelement 1A will be hereinafter referred to as a “first source S11,” a“first gate G11,” and a “first drain D11,” respectively, and the sourceS1, gate G1, and drain D1 of the second switching element 1B will behereinafter referred to as a “second source S12,” a “second gate G12,”and a “second drain D12,” respectively. Furthermore, in the followingdescription, out of the two control circuits 10, the control circuit 10associated with the first switching element 1A will be hereinafterreferred to as a “first control circuit 10 g 1” and the control circuit10 associated with the second switching element 1B will be hereinafterreferred to as a “second control circuit 10 g 2.” Furthermore, in thefollowing description, the inductor L1 of the first control circuit 10 g1 will be hereinafter referred to as a “first inductor L11” and theinductor L1 of the second control circuit 10 g 2 will be hereinafterreferred to as a “second inductor L12.” Furthermore, in the followingdescription, the drive circuit 2 associated with the first switchingelement 1A will be hereinafter referred to as a “first drive circuit 2A”and the drive circuit 2 associated with the second switching element 1Bwill be hereinafter referred to as a “second drive circuit 2B.”Furthermore, in the bidirectional switch including the two switchingelements 1, the electric current flowing from the first drain D11 towardthe second drain D12 will be hereinafter referred to as a “drain currentId1 d 2” and the electric current flowing from the second drain D12toward the first drain D11 will be hereinafter referred to as a “draincurrent Id2 d 1.”

In the switch device 100 g, the first control circuit 10 g 1 and thesecond control circuit 10 g 2 share the capacitor C1 as the circuitelement 5 and the first inductor L1 and the second inductor L2 areconnected in series. In the switch device 100 g, the first terminal ofthe first inductor L11 is connected to the first source S11 of the firstswitching element 1A, the first terminal of the second inductor L12 isconnected to the second source S12 of the second switching element 1B,and the second terminal of the first inductor L11 and the secondterminal of the second inductor L12 are connected to each other. In theswitch device 100 g, the capacitor C1 is connected between a node N15 onthe path between the second terminal of the first inductor L11 and thesecond terminal of the second inductor L12 and a node N3. The firstdrive circuit 2A is connected between the high-potential output terminalof the driver 3 and the first gate G11 of the first switching element1A. The second drive circuit 2B is connected between the high-potentialoutput terminal of the driver 3 and the second gate G12 of the secondswitching element 1B. In this variation, the second drive circuit 2B isconnected between a node N17, located on a path between thehigh-potential terminal of the driver 3 and the first drive circuit 2A,and the second gate G12 of the second switching element 1B. In thefollowing description, the low-potential output terminal (negative-sideterminal) of the driver 3 will be hereinafter referred to as thereference potential point P0 and the potential at the referencepotential point P0 will be hereinafter referred to as a “referencepotential Vstd” for the sake of convenience of description.

Next, the operation of the switch device 100 g will be described at thetime of turn OFF when the bidirectional switch is turned OFF from astate where the drain current Id1 d 2 is flowing through thebidirectional switch including the two switching elements 1 (i.e., whenthe two switching elements 1 are in ON state). As used herein, “to tunOFF the bidirectional switch” means turning OFF both the first switchingelement 1A and the second switching element 1B.

In the switch device 100 g, when the drain current Id1 d 2 that has beenincreasing starts to decrease after the bidirectional switch has startedto be turned OFF, counter electromotive force (induced electromotiveforce) is generated in each of the first inductor L11 and the secondinductor L12. In the switch device 100 g, when the counter electromotiveforce is generated in the first inductor L11, the reference potentialVstd becomes higher than the potential at the first source S11 of thefirst switching element 1A. As a result, the discharge current flowingfrom the first gate G11 of the first switching element 1A decreases,thus causing the cutoff rate of the drain current Id1 d 2 to slow downand thereby reducing the surge voltage applied to the switching element1A.

On the other hand, in the switch device 100 g, when the counterelectromotive force is generated in the second inductor L12, thereference potential Vstd becomes lower than the potential at the secondsource S12 of the second switching element 1B. As a result, thedischarge current flowing from the second gate G12 of the secondswitching element 1B increases, thus turning the second switchingelement 1B OFF before the first switching element 1A turns OFF. Whilethe drain current Id1 d 2 is flowing through the switch device 100 g,the second switching element 1B cannot cut off the drain current Id1 d2, no matter whether the second switching element 1B is ON or OFF. Thus,the cutoff rate of the drain current Id1 d 2 is not affected.

In the switch device 100 g, after the drain current Id1 d 2 has been cutoff, an electric current I9 flows, as a flow of the electric charge thathas been stored in, and drained from, the capacitor C1, through a firstclosed-loop circuit including the capacitor C1, the resistor R1, and thefirst inductor L11 in the first control circuit 10 g 1. In addition, anelectric current I10 flows, as a flow of the electric charge that hasbeen stored in, and drained from, the capacitor C1, through a secondclosed-loop circuit including the capacitor C1, the resistor R1, and thesecond inductor L12 in the second control circuit 10 g 2.

Next, the operation of the switch device 100 g will be described at thetime of turn OFF when the bidirectional switch is turned OFF from astate where the drain current Id2 d 1 is flowing through thebidirectional switch including the two switching elements 1 (i.e., whenthe two switching elements 1 are in ON state).

In the switch device 100 g, when the drain current Id2 d 1 that has beenincreasing starts to decrease after the bidirectional switch has startedto be turned OFF, counter electromotive force (induced electromotiveforce) is generated in each of the first inductor L11 and the secondinductor L12. In the switch device 100 g, when the counter electromotiveforce is generated in the second inductor L12, the reference potentialVstd becomes higher than the potential at the second source S12 of thesecond switching element 1B. As a result, the discharge current flowingfrom the second gate G12 of the second switching element 1B decreases,thus causing the cutoff rate of the drain current Id2 d 1 to slow downand thereby reducing the surge voltage.

On the other hand, in the switch device 100 g, when the counterelectromotive force is generated in the first inductor L11, thereference potential Vstd becomes lower than the potential at the firstsource S11 of the first switching element 1A. As a result, the dischargecurrent flowing from the first gate G11 of the second switching element1A increases, thus turning the first switching element 1A OFF before thesecond switching element 1B turns OFF. While the drain current Id2 d 1is flowing through the switch device 100 g, the first switching element1A cannot cut off the drain current Id2 d 1, no matter whether the firstswitching element 1A is ON or OFF. Thus, the cutoff rate of the draincurrent Id2 d 1 is not affected.

In the switch device 100 g, after the drain current Id2 d 1 has been cutoff, an electric current I9 flows, as a flow of the electric charge thathas been stored in, and drained from, the capacitor C1, through a firstclosed-loop circuit including the capacitor C1, the resistor R1, and thefirst inductor L11 in the first control circuit 10 g 1. In addition, anelectric current I10 also flows, as a flow of the electric charge thathas been stored in, and drained from, the capacitor C1, through a secondclosed-loop circuit including the capacitor C1, the resistor R1, and thesecond inductor L12 in the second control circuit 10 g 2.

As can be seen from the foregoing description, even the bidirectionalswitch device 100 g that uses a single source in common may also reducenot only the cutoff rate with respect to currents (Id1 d 2, Id2 d 1)flowing bidirectionally but also the surge voltage as well.

Thus, the switch device 100 g according to the second variation of thesecond embodiment would reduce a surge voltage applied to each of theswitching elements 1 while cutting down the switching loss involved wheneach of the switching elements if turns OFF.

Other Variations of Second Embodiment

In the second embodiment and the first and second variations thereof,the circuit element 5 is a capacitor C1. However, this configuration isonly an example and should not be construed as limiting. Alternatively,the circuit element 5 may also be a diode Di1 as well as the circuitelement 5 of the control circuit 10 b (see FIG. 5 ) according to thefirst variation of the first embodiment.

Also, in the foregoing description, the two basic circuits have the sameconfiguration. However, this configuration is only an example and shouldnot be construed as limiting. Alternatively, the circuit element 5 ofone of the two basic circuits may be the capacitor C1 and the circuitelement 5 of the other basic circuit may be the diode Di1. Stillalternatively, in the control circuit 10, two circuit elements 5 may beconnected in series to the inductor L1 with one circuit element 5implemented as the capacitor C1 and the other circuit element 5implemented as the diode Di1.

Optionally, each of the second embodiment and the first and secondvariations thereof may further include the protective diode Di2 (seeFIG. 7 ) of the control circuit 10 c according to the third variation ofthe first embodiment.

Optionally, each of the second embodiment and the first and secondvariations thereof may further include the protective diode Di3 of thecontrol circuit 10 d according to the fourth variation of the firstembodiment.

Third Embodiment

A control circuit 10 h according to a third embodiment and a switchdevice 100 h including the control circuit 10 h will be described withreference to FIG. 12 .

The control circuit 10 h according to the third embodiment includes, asthe circuit element 5, a resistor R1 s instead of the capacitor C1 ofthe control circuit 10 according to the first embodiment, which is adifference from the control circuit 10 according to the firstembodiment. The resistor R1 s is connected between the inductor L1 andthe low-potential output terminal (negative-side terminal) of the driver3. In the following description, the resistor R1 will be hereinafterreferred to as a “first resistor R1” and the resistor R1 s will behereinafter referred to as a “second resistor Rs1” for the sake ofconvenience of description.

Next, it will be described how the switch device 100 h including thiscontrol circuit 10 h operates.

In the switch device 100 h, when the source current Is that has beenincreasing starts to decrease, counter electromotive force (inducedelectromotive force) is generated in the inductor L1. When the counterelectromotive force is generated in the inductor L1, an electric currentflows through a closed-loop circuit including the inductor L1, thesecond resistor R1 s (circuit element 5), and the first resistor R1 inthe control circuit 10 h. Thus, in the switch device 100 h, thereference potential Vstd at the reference potential point P0 becomeshigher than the potential at the source S1 of the switching element 1.As a result, the potential difference between the potential at the gateG1 of the switching element 1 and the reference potential Vstddecreases, and therefore, the discharge current Idis flowing from thegate G1 of the switching element 1 also decreases, thus enabling cuttingoff the electric current gently (i.e., causing the cutoff rate of thesource current Is to slow down).

If the capacitor C1 is adopted as the circuit element 5 as in thecontrol circuit 10 according to the first embodiment, then the capacitorC1 is charged, thus causing the reference potential Vstd to changesignificantly. On the other hand, if the second resistor R1 s is adoptedas the circuit element 5 as in the control circuit 10 h according to thethird embodiment, the reference potential Vstd changes lesssignificantly than in a situation where the circuit element 5 is thecapacitor C1, thus achieving the advantages of making it easier topredict the operation of the control circuit 10 h and facilitating thecircuit design.

In addition, the switch device 100 h including the control circuit 10 haccording to the third embodiment may adjust the current variation rateof the principal current (source current) of the switching element 1 asthe ratio of the resistance value of the first resistor R1 to theresistance value of the second resistor R1 s, thus facilitating thedesign of the current variation rate. Furthermore, the second resistorR1 s has almost no capacitive component, thus reducing the chances of anegative bias being applied to the gate G1 of the switching element 1 asthe electric charge stored in the capacitive component is drained.

Optionally, the control circuit 10 h according to the third embodimentmay be implemented in combination with the control circuit 10 accordingto the first embodiment. That is to say, the control circuit 10 haccording to the third embodiment may also have a circuit configurationincluding not only the circuit element 5 (first circuit element)implemented as the first resistor R1 s but also a second circuit elementimplemented as the capacitor C1 connected in series to the first circuitelement.

Variation of Third Embodiment

In the third embodiment described above, the circuit element 5 isimplemented as the resistor R1 s. However, this configuration is only anexample and should not be construed as limiting. Specifically, as shownin FIG. 13 , a control circuit 10 i according to a variation of thethird embodiment includes not only the circuit element 5 implemented asthe resistor R1 s (first circuit element) but also a circuit element 5implemented as a diode Dis (second circuit element). In the followingdescription, any constituent element of the control circuit 10 i andswitch device 100 i according to this variation of the third embodiment,having the same function as a counterpart of the control circuit 10 hand switch device 100 h according to the third embodiment describedabove, will be designated by the same reference numeral as thatcounterpart's, and description thereof will be omitted as appropriateherein.

In the control circuit 10 i included in the switch device 100 i, theresistor R1 s and the diode Dis are connected in series. In thisvariation, the cathode of the diode Dis is connected to the resistor R1s and the anode of the diode Dis is connected to the node N2. Thus, inthis control circuit 10 i, the resistor R1 is connected in parallel to aseries circuit of the resistor R1 s (first circuit element) and thediode Dis (second circuit element). In this switch device 100 i, as wellas in the switch device 100, the high-potential output terminal(positive-side terminal) of the driver 3 is connected to the gate G1 ofthe switching element 1 via the drive circuit 2.

Next, it will be described how the switch device 100 i including thecontrol circuit 10 i operates.

In the control circuit 10 i, when the source current Is that has beenincreasing starts to decrease when the switching element 1 turns OFF,electromotive force (counter electromotive force) is generated in theinductor L1. In the control circuit 10 i, when the counter electromotiveforce is generated in the inductor L1, an electric current flows througha closed-loop circuit including the inductor L1, the diode Dis, thesecond resistor R1 s, and the first resistor R1. Thus, in the switchdevice 100 i, the reference potential Vstd at the reference potentialpoint P0 becomes higher than the potential at the source S1. As aresult, in the switch device 100 i, the potential difference between thepotential at the gate G1 of the switching element 1 and the referencepotential Vstd decreases, and therefore, the discharge current Idisflowing from the gate G1 of the switching element 1 also decreases, thusenabling cutting off the source current Is of the switching element 1gently.

In addition, the switch device 100 i including the control circuit 10 iaccording to this variation of the third embodiment may adjust thecurrent variation rate of the principal current (source current) of theswitching element 1 as the ratio of the resistance value of the firstresistor R1 to the resistance value of the second resistor R1 s, thusfacilitating the design of the current variation rate. Furthermore, thesecond resistor R1 s and the diode Dis have almost no capacitivecomponent, thus reducing the chances of a negative bias being applied tothe gate G1 of the switching element 1 as the electric charge stored inthe capacitive component is drained.

Optionally, the control circuit 10 i according to this variation of thethird embodiment may be implemented in combination with the controlcircuit 10 according to the first embodiment. That is to say, thecontrol circuit 10 i may include a plurality of circuit elements 5 thatare connected in series to each other between the nodes N2, N3. Forexample, the control circuit 10 i may include a series circuit includingthe second resistor R1 s (first circuit element), the diode Dis (secondcircuit element), and the capacitor C1 (third circuit element). Whenelectromotive force is generated in the inductor L1, an electric currentflows through the first, second, and third circuit elements.

Fourth Embodiment

A control circuit 10 j according to a fourth embodiment and a switchdevice (switch system) 100 j including the control circuit 10 j will bedescribed with reference to FIG. 14 .

The control circuit 10 j according to the fourth embodiment issubstantially the same as the control circuit 10 h according to thethird embodiment (see FIG. 12 ). The control circuit 10 j furtherincludes a voltage clamping element 9, which is connected in parallel tothe switching element 1 and the inductor L1 (hereinafter referred to asa “first inductor Ls1”), which is a difference from the control circuit10 h according to the third embodiment. In the following description,any constituent element of the control circuit 10 j and switch device100 j according to the fourth embodiment, having the same function as acounterpart of the control circuit 10 h and switch device 100 haccording to the third embodiment described above, will be designated bythe same reference numeral as that counterpart's, and descriptionthereof will be omitted herein.

The voltage clamping element 9 has an overvoltage protection function oflimiting the surge voltage applied to the switching element 1 when theswitching element 1 turns OFF to a predetermined voltage (clampvoltage). That is to say, the voltage clamping element 9 has thefunction of limiting the voltage between the drain D1 and source S1 ofthe switching element 1 to a predetermined voltage when the switchingelement 1 turns OFF. In the example shown in FIG. 14 , the voltageclamping element 9 is a varistor. However, this is only an example andshould not be construed as limiting. Alternatively, the voltage clampingelement 9 may also be a Zener diode (such as a TVS diode). The voltageclamping element 9 has the function of reducing, when a voltage equal toor higher than a certain voltage is applied, the chances of the voltageincreasing to a higher voltage. In the meantime, an electric currentflows through the voltage clamping element 9.

In addition, the control circuit 10 j further includes a second inductorLs2 and a third inductor Ls3. The second inductor Ls2 is connectedbetween the first inductor Ls1 and the second resistor R1 s as thecircuit element 5. The third inductor Ls3 is connected between thevoltage clamping element 9 and the path between the second inductor Ls2and the circuit element 5. Thus, in the switch device 100 j, a seriescircuit of the voltage clamping element 9, the third inductor Ls3, andthe second inductor Ls2 is connected in parallel to a series circuit ofthe switching element 1 and the first inductor Ls1. In the controlcircuit 10 j, the sum of the inductance of the first inductor Ls1 andthe inductance of the second inductor Ls2 is greater than the inductanceof the third inductor Ls3.

The switch device 100 j further includes, for example, a first terminalT1, to which the drain D1 of the switching element 1 is connected, and asecond terminal T2, to which a second terminal of the inductor L1 isconnected. A first terminal of the inductor L1 is connected to thesource S1 of the switching element 1. That is to say, in the switchdevice 100 j, a series circuit of the switching element 1 and the firstinductor Ls1 is connected between the first terminal T1 and the secondterminal T2. Also, in this switch device 100 j, a load circuit includinga load and a power supply, for example, is connected between the firstterminal T1 and the second terminal T2, thus connecting the load circuitto the series circuit of the switching element 1 and the first inductorLs1. Note that the load and the power supply are not constituentelements of the switch device 100 j.

In the switch device 100 j, the first terminal T1 and the secondterminal T2 are terminals through which a principal current (sourcecurrent Is) flowing through the switching element 1 flows when theswitching element 1 is electrically conductive. One terminal of thesecond resistor R1 s of the control circuit 10 j is connected to a nodeN10 located on the path between the voltage clamping element 9 and thesecond terminal T2. The node N10 is located on the path through whichthe gate current of the switching element 1 flows when the switchingelement 1 is switched in the switch device 100 j. The node N10 is alsolocated on the path through which the source current Is does not flowwhile the switching element 1 is electrically conductive.

Next, it will be described how the switch device 100 j including thecontrol circuit 10 j operates.

In the switch device 100 j, when the source current Is of the switchingelement 1 that has been increasing starts to decrease when the switchingelement 1 turns OFF, electromotive force (counter electromotive force)is generated in the first inductor Ls1. At this time, inducedelectromotive force is also generated in a parasitic inductance such asa wire of the load circuit connected between the first terminal T1 andthe second terminal T2. Nevertheless, if the voltage exceeds the clampvoltage of the voltage clamping element 9, a further increase in thevoltage is checked by the voltage clamping element 9.

On the other hand, in the switch device 100 j, when the voltage clampingelement 9 is activated, an electric current flows from the firstterminal T1 to the second terminal T2 via the third inductor Ls3, thenode N10, and the second inductor Ls2. This electric current generateselectromotive force in each of the second inductor Ls2 and the thirdinductor Ls3. Thus, in the control circuit 10 j, an electric currentflows through a closed-loop circuit including the first inductor Ls1,the second inductor Ls2, the second resistor R1 s, and the firstresistor R1. Consequently, in the switch device 100 j, the referencepotential Vstd at the reference potential point P0 becomes higher thanthe potential at the source S1 of the switching element 1, the potentialdifference between the potential at the gate G1 of the switching element1 and the reference potential Vstd decreases, and therefore, thedischarge current Idis flowing from the gate G1 of the switching element1 also decreases, thus enabling cutting off the source current Isgently.

The control circuit 10 j according to the fourth embodiment includes thefirst inductor Ls1 and the second inductor Ls2 instead of the inductorL1 of the control circuit 10 h (see FIG. 12 ) according to the thirdembodiment. In the control circuit 10 h, the induced electromotive force(counter electromotive force), generated in the inductor L1 as thesource current Is decreases, increases as the inductance of the inductorL1 increases. From a different point of view, if the inductor L1 hassignificant inductance, significant electromotive force is generatedeven if the current variation rate has a small absolute value when thesource current Is decreases. Thus, the control circuit 10 j according tothe fourth embodiment achieves the advantage of broadening the operatingrange of the control circuit 10 j with respect to the current variationrate of the source current Is. It is sometimes easy for the controlcircuit 10 j according to the fourth embodiment to increase theinductance of the second inductor Ls2. In the control circuit 10 j,while the switching element 1 is ON state (i.e., electricallyconductive), an electric current flows continuously through the firstinductor Ls1. Thus, if heat generation is a problem, the conductorportion that forms the first inductor Ls1 preferably has its width ordiameter increased. On the other hand, the second inductor Ls2 is aportion through which an electric current flows only for a certainperiod of time when the switching element 1 turns OFF and the voltageclamping element 9 is activated. The second inductor Ls2 rarely causesthe problem of heat generation. Thus, the conductor portion that formsthe second inductor Ls2 may have its width and/or diameter decreased.Therefore, it is the second inductor Ls2 that its size and cost hardlyincrease when the inductance is increased. The control circuit 10 jaccording to the fourth embodiment achieves the advantage of making iteasier to broaden the operating range of the control circuit 10 j withrespect to the current variation rate when the source current Isdecreases by increasing the inductance of the second inductor Ls2.

Also, the induced electromotive force generated in the third inductorLs3 in the control circuit 10 j is superposed on the clamp voltage ofthe voltage clamping element 9 and applied to the switching element 1.Thus, to reduce the surge voltage applied to the switching element 1,the ratio of the sum of the respective inductances of the first inductorLs1 and the second inductor Ls2 to the inductance of the third inductorLs3 is preferably as large as possible.

The first inductor Ls1, the second inductor Ls2, and the third inductorLs3 do not have to be electronic components but may each be a conductorpattern (such as a copper pattern) on a board, an electric wire cable,or a lead wire of the voltage clamping element 9, for example.

Fifth Embodiment

A switch device (switch system) 100 k according to a fifth embodimentwill be described with reference to FIG. 15 .

The switch device 100 k according to the fifth embodiment includes aswitching element 1 k instead of the switching element 1 of the switchdevice 100 j according to the fourth embodiment and includes two controlcircuits 10 j, which is a difference from the switch device 100 jaccording to the fourth embodiment. The switching element 1 k is adual-gate bidirectional switch including two gates G1 and two sourcesS1.

In the switching element 1 k, the two gates G1 and the two sources S1correspond one to one to each other. In the following description, oneof the two gates G1 will be hereinafter referred to as a “first gateG11” and the other gate G1 as a “second gate G12” for the sake ofconvenience of description. In the same way, out of the two sources S1,the source S1 corresponding to the first gate G11 will be hereinafterreferred to as a “first source S11” and the source S1 corresponding tothe second gate G12 will be hereinafter referred to as a “second sourceS12.” The switching element 1 k has the same configuration as theswitching element 1 f (see FIG. 10 ).

In the switch device 100 k according to the fifth embodiment, out of thetwo control circuits 10 j, one control circuit 10 j is connected betweenthe first gate G11 and first source S11 of the switching element 1 k andthe other control circuit 10 j is connected between the second gate G12and the second source S12 of the switching element 1 k. Furthermore, inthe switch device 100 k, the voltage clamping element 9 is used incommon by the two control circuits 10 j and the voltage clamping element9 is connected between the two third inductors Ls3.

The switch device 100 k according to the fifth embodiment may reduce asurge voltage applied to the switching element 1 k while cutting downthe switching loss involved when the switching element 1 k turns OFF.

Note that the first to fifth embodiments and their variations describedabove are only exemplary ones of various embodiments and theirvariations of the present disclosure and should not be construed aslimiting. Rather, the first to fifth exemplary embodiment and theirvariations may be readily modified in various manners depending on adesign choice or any other factor without departing from the scope ofthe present disclosure.

For example, even though the control circuit 10 includes neither thedrive circuit 2 nor the driver 3, the control circuit 10 may include atleast one of the drive circuit 2 or the driver 3. Also, in the switchdevice 100, the driver 3 may include the drive circuit 2.

The first to fifth embodiments and their variations described above maybe specific implementations of the following aspects of the presentdisclosure.

A control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e 1; 10 e 2; 10 f 1;10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) according to a first aspect isa control circuit for controlling a switching element (1; 1 f; 1 k)including a gate (G1) and a source (S1) corresponding to the gate (G1).The control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e 1; 10 e 2; 10 f 1;10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) includes an inductor (L1), acircuit element (5), and a resistor (R1). The inductor (L1) is connectedbetween the gate (G1) and the source (S1) of the switching element (1; 1f; 1 k). The circuit element (5) is connected in series to the inductor(L1) between the gate (G1) and the source (S1). The circuit element (5)allows an electric current to flow therethrough in response togeneration of electromotive force in the inductor (L1). The resistor(R1) is connected in parallel to the inductor (L1) and the circuitelement (5) between the gate (G1) and the source (S1).

This configuration may be expected to reduce a surge voltage applied toa switching element (1; 1 f; 1 k) while cutting down the switching lossinvolved when the switching element (1; 1 f; 1 k) turns OFF.

In a control circuit (10; 10 a) according to a second aspect, which maybe implemented in conjunction with the first aspect, the circuit element(5) includes a capacitor (C1).

This configuration enables changing the current variation rate of aprincipal current (source current Is) flowing through the switchingelement (1) by changing the circuit constant of the capacitance of thecapacitor (C1).

In a control circuit (10 b) according to a third aspect, which may beimplemented in conjunction with the first aspect, the circuit element(5) includes a diode (Di1).

This configuration enables reducing the amount of an electric currentdischarged from the circuit element (5) after the principal current(source current Is) has been cut off, compared to the control circuit(10; 10 a) according to the second aspect.

In a control circuit (10 h) according to a fourth aspect, which may beimplemented in conjunction with the first aspect, the circuit element(5) includes a resistor (R1 s).

This configuration makes it easier to design the current variation rateof a principal current (source current Is) flowing through the switchingelement (1; 1 f; 1 k) compared to the control circuit (10; 10 a)according to the second aspect and the control circuit (10 b) accordingto the third aspect. According to this configuration, the currentvariation rate is determined by a ratio of the resistance value of theresistor (R1) to the resistance value of the resistor (R1 s). Inaddition, in the control circuit (10 h) according to the fourth aspect,no discharge current flows from the circuit element (5) after theprincipal current of the switching element (1; 1 f; 1 k) has been cutoff, thus enabling protecting the switching element (1; 1 f; 1 k).

In a control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e 1; 10 e 2; 10 f1; 10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) according to a fifthaspect, which may be implemented in conjunction with any one of thefirst to fourth aspects, when the switching element (1; 1 f; 1 k) turnsOFF, an electric current flowing through the source (S1) decreases togenerate electromotive force in the inductor (L1). As an electriccurrent corresponding to the electromotive force flows through thecircuit element (5) and the resistor (R1), a potential at a referencepotential point (P0) included in a path between a point of connectionwhere the circuit element (5) and the resistor (R1) are connectedtogether and the gate (G1) increases. A discharge current (Idis) flowingfrom the gate (G1) is determined by a potential difference between apotential at the gate (G1) and the potential (Vstd) at the referencepotential point (P0).

According to this configuration, the current (Idis) flowing from thegate (G1) is determined by the potential difference between thepotential at the gate (G1) and the potential (Vstd) at the referencepotential point (P0). Thus, the discharge current (Idis) may be limitedby causing an increase in the potential (Vstd) at the referencepotential point (P0).

A control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e 1; 10 e 2; 10 f 1;10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) according to a sixth aspect,which may be implemented in conjunction with any one of the first tofifth aspects, further includes a protective diode (Di2). The protectivediode (Di2) includes an anode and a cathode. The anode is connected to apoint of connection (node N3) between the circuit element (5) and theresistor (R1). The cathode is connected to the gate (G1) of theswitching element (1; 1 f; 1 k).

This configuration enables protecting the switching element (1; 1 f; 1k).

A control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e 1; 10 e 2; 10 f 1;10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) according to a seventh aspect,which may be implemented in conjunction with any one of the first tofifth aspects, further includes a protective diode (Di3). The protectivediode (Di3) includes an anode and a cathode. The anode is connectedbetween the source (S1) of the switching element (1; 1 f; 1 k) and anode (N1) located between the inductor (L1) and the resistor (R1). Thecathode is connected to the gate (G1) of the switching element (1; 1 f;1 k).

This configuration enables protecting the switching element (1; 1 f; 1k).

A control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e 1; 10 e 2; 10 f 1;10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) according to an eighth aspect,which may be implemented in conjunction with any one of the first toseventh aspects, further includes a first terminal (T1), a secondterminal (T2), a second inductor (Ls2), a voltage clamping element (9),and a third inductor (Ls3). The first terminal (T1) is connected to theswitching element (1; 1 f; 1 k) at a point, located opposite from thesource (S1), of the switching element (1; 1 f; 1 k). The second terminal(T2) is connected to the inductor (L1) at a point, located opposite fromthe switching element (1; 1 f; 1 k), of the inductor (L1). The secondinductor (Ls2) is connected between a first node (node N2) and thecircuit element (5). The first node (node N2) is located between a firstinductor (Ls1) serving as the inductor (L1) and the second terminal(T2). The voltage clamping element (9) is connected in parallel to theswitching element (1; 1 f; 1 k), the first inductor (Ls1), and thesecond inductor (Ls2). The third inductor (Ls3) is connected between asecond node (node N10) and the voltage clamping element (9). The secondnode (node N10) is located between the second inductor (Ls2) and thecircuit element (5). In this control circuit (10; 10 a; 10 b; 10 c; 10d; 10 e 1; 10 e 2; 10 f 1; 10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j), noelectric current flows through the third inductor (Ls3) while theswitching element (1; 1 f; 1 k) is in ON state.

This configuration enables protecting the switching element (1; 1 f; 1k). In addition, this configuration also makes it easier to set a broadoperating range of the control circuit (10; 10 a; 10 b; 10 c; 10 d; 10 e1; 10 e 2; 10 f 1; 10f2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j) with respectto a current variation rate when an electric current (source current Is)flowing through the source (S1) of the switching element (1; 1 f; 1 k)decreases.

A switch device (100; 100 a; 100 b; 100 c; 100 d; 100 e 1; 100 e 2; 100f 1; 100 f 2; 100 g 1; 100 g 2; 100 h; 100 i; 100 j) according to aninth aspect includes the control circuit (10; 10 a; 10 b; 10 c; 10 d;10 e 1; 10 e 2; 10 f 1; 10 f 2; 10 g 1; 10 g 2; 10 h; 10 i; 10 j)according to any one of the first to eighth aspects and the switchingelement (1; 1 f; 1 k).

This configuration may be expected to reduce a surge voltage applied toa switching element (1; 1 f; 1 k) while cutting down the switching lossinvolved when the switching element (1; 1 f; 1 k) turns OFF.

A switch device (100; 100 a; 100 b; 100 c; 100 d; 100 e; 100 g; 100 h;100 i) according to a tenth aspect, which may be implemented inconjunction with the ninth aspect, includes two switching elements (1)and two control circuits (10; 10 a; 10 b; 10 c; 10 d; 10 h; 10 i). Inthe switch device (100; 100 a; 100 b; 100 c; 100 d; 100 e; 100 g; 100 h;100 i), the two switching elements (1) are connected in series. The twocontrol circuits (10; 10 a; 10 b; 10 c; 10 d; 10 h; 10 i) are associatedone to one with the two switching elements (1).

This configuration may be expected to reduce a surge voltage applied toa switching element (1) while cutting down the switching loss involvedwhen the two switching elements (1) turn OFF.

In a switch device (100; 100 a; 100 b; 100 c; 100 d; 100 e; 100 f; 100g) according to an eleventh aspect, which may be implemented inconjunction with the tenth aspect, each of the two switching elements(1) includes a drain (D1) corresponding to the gate (G1). The respectivedrains (D1) of the two switching elements (1) are connected to eachother.

This configuration may be expected to reduce a surge voltage applied tothe two switching elements (1) while cutting down the switching lossinvolved when the two switching elements (1) turn OFF.

In a switch device (100 f; 100 k) according to a twelfth aspect, whichmay be implemented in conjunction with the ninth aspect, the switchingelement (1 f; 1 k) is a dual-gate bidirectional switch including twogates (G1) and two sources (S1). The switch device (100 f; 100 k)includes two control circuits (10; 10 j). One control circuit out of thetwo control circuits (10; 10 j) is connected to a gate (G1), associatedwith the one control circuit, out of the two gates (G1) of thebidirectional switch. The other control circuit out of the two controlcircuits (10; 10 j) is connected to a gate (G1), associated with theother control circuit, out of the two gates (G1) of the bidirectionalswitch.

This configuration may be expected to reduce a surge voltage applied tothe switching element (1 f; 1 k) while cutting down the switching lossinvolved when the switching element (1 f; 1 k), implemented as adual-gate bidirectional switch, turns OFF.

In a switch device (100 g) according to a thirteenth aspect, which maybe implemented in conjunction with the tenth aspect, the respectivesources (S1, S2) of the two switching elements (1) are connected to eachother.

This configuration may be expected to reduce a surge voltage applied totwo switching elements (1) while cutting down the switching lossinvolved when the two switching elements (1) turn OFF.

An object of the present disclosure to be described below is to providea control circuit and a switch system that may reduce a surge voltageapplied to a semiconductor switch while cutting down the switching lossinvolved when the semiconductor switch turns OFF.

FIRST EXAMPLE

A control circuit 12 according to a first example and a switch system 13including the control circuit 12 will be described with reference toFIGS. 16 and 17 .

(1) Overview

The control circuit 12 is a control circuit for controlling asemiconductor switch 11. The semiconductor switch 11 includes a gate 11Gand a source 11S corresponding to the gate 11G. The semiconductor switch11 further includes a drain 11D in addition to the gate 11G and thesource 11S. The control circuit 12 includes, as discharge paths throughwhich electric charge is drained from the gate 11G of the semiconductorswitch 11, a first discharge path 21 and a second discharge path 22,which allows the electric charge to be drained more rapidly than thefirst discharge path 21. The control circuit 12 includes a first switchQ11 and a second switch Q12 which are provided for the second dischargepath 22. The second switch Q12 selectively turns ON according to thecurrent variation rate of a principal current I_(DS) (see FIG. 19 ) ofthe semiconductor switch 11. The principal current I_(DS) of thesemiconductor switch 11 is an electric current flowing from the drain11D of the semiconductor switch 11 to the source 11S thereof. Thecontrol circuit 12 includes, as a current variation rate detection unit23 (see FIG. 16 ) for detecting a current variation rate, an inductor Ls(see FIG. 17 ) connected to the source 11S of the semiconductor switch11, for example.

The switch system 13 includes the control circuit 12 and thesemiconductor switch 11. Also, in this switch system 13, a seriescircuit of a load 15 and a power supply 16 is connected between thedrain 11D and source 11S of the semiconductor switch 11, for example. Inthe switch system 13, the series circuit of the load 15 and the powersupply 16 is connected to the series circuit of the semiconductor switch11 and the inductor Ls. Note that the load 15 and the power supply 16are not counted among the constituent elements of the switch system 13.

(2) Respective Constituent Elements of Switch System (2.1) SemiconductorSwitch

The semiconductor switch 11 is, for example, a GaN-based semiconductorswitch. More specifically, the semiconductor switch 11 may be a junctionfield effect transistor (JFET). The JFET serving as the semiconductorswitch 11 is, for example, a GaN-based gate injection transistor (GIT).

The semiconductor switch 11 includes, for example, a substrate, a bufferlayer, a first nitride semiconductor layer, a second nitridesemiconductor layer, a source electrode, a gate electrode, a drainelectrode, and a p-type layer. The buffer layer is formed on thesubstrate. The first nitride semiconductor layer is formed on the bufferlayer. The second nitride semiconductor layer is formed on the firstnitride semiconductor layer. The source electrode, the gate electrode,and the drain electrode are formed on the second nitride semiconductorlayer. The p-type layer is interposed between the gate electrode and thesecond nitride semiconductor layer. In the semiconductor switch 11, adiode structure is formed by the second nitride semiconductor layer andthe p-type layer. The gate 11G of the semiconductor switch 11 includesthe gate electrode and the p-type layer. The source 11S of thesemiconductor switch 11 includes the source electrode. The drain 11D ofthe semiconductor switch 11 includes the drain electrode. The substrateis a silicon substrate, for example. The buffer layer is an undoped GaNlayer, for example. The first nitride semiconductor layer is, forexample, an undoped GaN layer. The second nitride semiconductor layeris, for example, an undoped AlGaN layer. The p-type layer is, forexample, a p-type AlGaN layer. Each of the buffer layer, the firstnitride semiconductor layer, and the second nitride semiconductor layermay include impurities such as Mg, H, Si, C, and O to be inevitablycontained during their growing process by metal-organic vapor phaseepitaxy (MOVPE), for example.

(2.2) Control Circuit (2.2.1) Configuration of Control Circuit

As shown in FIG. 17 , the control circuit 12 according to the firstexample includes the first discharge path 21, the second discharge path22, the first switch Q11, and the second switch Q12. The first dischargepath 21 is connected to the gate 11G of the semiconductor switch 11. Thesecond discharge path 22 is connected to the gate 11G of thesemiconductor switch 11. The second discharge path 22 enables a morerapid discharge than the first discharge path 21. The second switch Q12may be turned ON and OFF separately from the first switch Q11. Thesecond switch Q12 is provided on the second discharge path 22 andselectively turns ON according to the current variation rate of aprincipal current of the semiconductor switch 11. In this case, in thecontrol circuit 12 according to the first example, the second switch Q12turns ON with the electromotive force to be generated in the inductor Lsaccording to the current variation rate.

(2.2.2) Details of Control Circuit

As shown in FIG. 17 , the control circuit 12 includes the firstdischarge path 21, the second discharge path 22, the first switch Q11,and the second switch Q12.

In this control circuit 12, the first discharge path 21 and the seconddischarge path 22 include a common discharge path 20 connected to thegate 11G of the semiconductor switch 11. The semiconductor switch 11 isa normally OFF semiconductor switch.

The first discharge path 21 includes a gate resistor R_(G) connected tothe gate 11G of the semiconductor switch 11. The gate resistor R_(G) isprovided for a part, except the common discharge path 20, of the firstdischarge path 21. The first discharge path 21 is a path for reducingthe absolute value of the current variation rate (−dI_(DS)/dt) of theprincipal current I_(DS) when the semiconductor switch 11 turns OFF.

The second discharge path 22 is connected to the gate 11G of thesemiconductor switch 11 not via the gate resistor R_(G). The seconddischarge path 22 is a path allowing the electric charge stored in thegate 11G of the semiconductor switch 11 to be drained more rapidly thanthe first discharge path 21.

The first switch Q11 and the second switch Q12 are provided on thesecond discharge path 22.

The first switch Q11 is connected to a node N11 between the gateresistor R_(G) and the gate 11G of the semiconductor switch 11. Thefirst switch Q11 is a p-channel field effect transistor Tr1 provided onthe second discharge path 22. In this case, the p-channel field effecttransistor Tr1 includes a gate, a source, and a drain. In the exampleillustrated in FIG. 17 , the field effect transistor Tr1 is a normallyOFF p-channel MOSFET. Meanwhile, the second switch Q12 is a diode D2provided on the second discharge path 22. The diode D2 includes an anodeand a cathode.

In the control circuit 12, the source of the p-channel field effecttransistor Tr1 is connected to the gate 11G of the semiconductor switch11 and the drain of the p-channel field effect transistor Tr1 isconnected to the anode of the diode D2. Also, in this control circuit12, the gate resistor R_(G) is connected between the gate and source ofthe p-channel field effect transistor Tr1.

The second discharge path 22 includes an inductor Ls which is connectedin series to the diode D2. Thus, on the second discharge path 22, thep-channel field effect transistor Tr1, the diode D2, and the inductor Lsare connected in series. The inductor Ls has a first terminal and asecond terminal. On the second discharge path 22, the first terminal ofthe inductor Ls is connected to the cathode of the diode D2. On thesecond discharge path 22, the second terminal of the inductor Ls isconnected to the source 11S of the semiconductor switch 11. The secondswitch Q12 is provided on the second discharge path 22 as describedabove and selectively turns ON according to the current variation rateof the principal current I_(DS) of the semiconductor switch 11. In thecontrol circuit 12 according to the first example, the second switch Q12turns ON in accordance with the electromotive force generated in theinductor Ls in response to a current variation of the principal currentI_(DS).

In this control circuit 12, a driver 14 is connected via the gateresistor R_(G) between the node N11 and the second terminal of theinductor Ls. The driver 14 is not a constituent element of the controlcircuit 12 but a constituent element of the switch system 13. The driver14 has a high-potential output terminal and a low-potential outputterminal. In this control circuit 12, the high-potential output terminalof the driver 14 is connected to the gate resistor R_(G) and thelow-potential output terminal of the driver 14 is connected to thesecond terminal of the inductor Ls. In the switch system 13, thelow-potential output terminal of the driver 14 is connected to a nodeN12 between the source 11S of the semiconductor switch 11 and the secondterminal of the inductor Ls. The driver 14 is a driver which may applynot only a positive bias voltage but also a negative bias voltage tobetween the gate 11G and source 11S of the semiconductor switch 11. Thedriver 14 is a driver which includes, for example, a DC power supply anda complementary metal-oxide semiconductor (CMOS) inverter and which maychange the output voltage within the range from −12 V to 18 V.

(2.2.3) Operation of Control Circuit and Switch System

Next, it will be described with reference to FIGS. 18, 19A, 19B, 20A,20B, 21A, and 21B how the control circuit 12 and the switch system 13operate. Note that in FIGS. 18, 19A, 20A, and 21A, a circuit section,through which no electric current flows, is drawn in a different type ofline from the other circuit sections to make the former circuit sectioneasily recognizable.

In the switch system 13, while a positive bias voltage is output fromthe driver 14 to between the gate 11G and source 11S of thesemiconductor switch 11 (note that the driver 14 is represented by a DCpower supply E4 in FIG. 18 ), the semiconductor switch 11 is in ONstate. At this time, in the p-channel field effect transistor Tr1, thepotential at the gate is higher than the potential at the source, andtherefore, the p-channel field effect transistor Tr1 is not electricallyconductive.

To turn the semiconductor switch 11 OFF, the switch system 13 changesthe output voltage of the driver 14 from a positive bias voltage into 0V, for example, (or a negative bias voltage). As a result, the drain11D-source 11S voltage V_(DS), the principal current I_(DS), and gate11G-source 11S voltage V_(GS) of the semiconductor switch 11 vary asshown in FIGS. 19B-21B.

FIG. 19A illustrates how the control circuit 12 and the switch system 13h operate in the period from a point in time t0 to a point in time t2shown in FIG. 19B (i.e., the period indicated by dot hatching in FIG.19B). In FIG. 19B, t0 is the point in time when the switch system 13 hchanges the output voltage of the driver 14 from a positive bias voltageinto, for example, 0 V (or a negative bias voltage) and t2 is a point intime when the drain 11D-source 11S voltage V_(DS) of the semiconductorswitch 11 finishes increasing. In the period from the point in time t0through the point in time t2 shown in FIG. 19B, the first switch Q11 isON and the second switch Q12 is ON, and therefore, the gate currentI_(G) is drained through the first switch Q11 and the second switch Q12.That is to say, the electric charge stored in the gate 11G of thesemiconductor switch 11 is drained through the second discharge path 22.Thus, in the gate current I_(G), the current I_(Q11) flowing through thefirst switch Q11 is dominant More specifically, in the period from thepoint in time t0 to the point in time t1 before the drain 11D-source 11Svoltage V_(DS) of the semiconductor switch 11 starts to increase, theelectric charge in the gate 11G of the semiconductor switch 11 isdrained rapidly, thus causing a steep decrease in the gate 11G-source11S voltage V_(GS) of the semiconductor switch 11. Then, once the drain11D-source 11S voltage V_(DS) of the semiconductor switch 11 has startedto increase from the point in time t1, the gate 11G-source 11S voltageV_(GS) becomes substantially constant.

FIG. 20A illustrates how the control circuit 12 and the switch system 13h operate in the period from a point in time t2 to a point in time t3shown in FIG. 20B (i.e., the period indicated by dot hatching in FIG.20B). In the switch system 13, in the period from the point in time t2to the point in time t3, the drain 11D-source 11S voltage V_(DS) of thesemiconductor switch 11 is substantially constant from the point in timet2 on as shown in FIG. 20B. As the principal current I_(DS) starts todecrease from the point in time t2, electromotive force is generatedbetween the first terminal and second terminal of the inductor Ls due tothe variation in the principal current I_(DS) to cause the diode D2 toturn OFF. As a result, the current I_(Q11) flowing through the p-channelfield effect transistor Tr1 decreases, and therefore, the gate currentI_(G) flows through the gate resistor R_(G). That is to say, theelectric charge in the gate 11G of the semiconductor switch 11 starts tobe drained through the first discharge path 21, instead of the seconddischarge path 22. Thus, the magnitude of the gate current I_(G) isdetermined by the resistance value of the gate resistor R_(G). Theresistance value of the gate resistor R_(G) falls, for example, withinthe range from 50 Ω to 5 kΩ. If the resistance value of the gateresistor R_(G) is a relatively large value (e.g., 3 kΩ or more), thenthe current variation rate dI_(DS)/dt has a value derived by thefollowing Equation (1)

L1×dI _(DS) /dt=V _(GS) −V _(thD2)   (1)

where L1 is the inductance of the inductor Ls and V_(thD2) is athreshold voltage at which the diode D2 turns ON.

FIG. 21A illustrates how the control circuit 12 and the switch system 13operate in the period from a point in time t3 to a point in time t4shown in FIG. 21B (i.e., the period indicated by dot hatching in FIG.21B). In the switch system 13, as shown in FIG. 21A, when the principalcurrent I_(DS) of the semiconductor switch 11 becomes approximatelyequal to zero at the point in time t3, no electromotive force isgenerated in the inductor Ls any longer and the second switch Q12 turnsON. Thus, the gate current I_(G) starts to flow through the seconddischarge path 22 instead of the first discharge path 21. That is tosay, in the gate current I_(G), the current I_(Q1) flowing through thefirst switch Q11 becomes dominant Thus, the electric charge in the gateof the semiconductor switch 11 is drained rapidly through the seconddischarge path 22. Consequently, the gate 11G-source 11S voltage V_(GS)of the semiconductor switch 11 decreases steeply to become approximatelyequal to zero at the point in time t4.

(3) Characteristics of Semiconductor Switch to be Controlled by ControlCircuit

FIG. 22 shows the characteristics of the semiconductor switch 11 in asituation where the resistance value of the gate resistor R_(G) ischanged within the range from 100 Ω to 5 kΩ in the control circuit 12.In this case, the characteristics of the semiconductor switch 11 arerepresented by the variations with time in the gate 11G-source 11Svoltage V_(GS), principal current I_(DS), and drain 11D-source 11Svoltage V_(DS) of the semiconductor switch 11. In FIG. 22 , fivecharacteristics of the semiconductor switch 11 are shown and areindicated by the reference signs A1, A2, A3, A4, and A5 in the ascendingorder of the resistance value of the gate resistor R_(G). Specifically,in FIG. 22 , A1 indicates the characteristic when the resistance valueof the gate resistor R_(G) is the smallest, and A5 indicates thecharacteristic when the resistance value of the gate resistor R_(G) isthe largest.

As can be seen from the results shown in FIG. 22 , the control circuit12 may change the current variation rate of the principal current I_(DS)of the semiconductor switch 11 by varying the resistance value of thegate resistor R_(G) and may decrease the absolute value of the currentvariation rate by increasing the resistance value. It can also be seenfrom the results shown in FIG. 22 that the control circuit 12 may reducethe respective oscillations of the gate 11G-source 11S voltage V_(GS),principal current I_(DS), and drain 11D-source 11S voltage V_(DS) of thesemiconductor switch 11 by increasing the resistance value of the gateresistor R_(G). In addition, it can also be seen from the results shownin FIG. 22 that the control circuit 12 may reduce, by applying anegative bias voltage between the gate 11G and the source 11S of thesemiconductor switch 11, the chances of the gate 11G-source 11S voltageV_(GS) exceeding the threshold voltage to cause a false turn ON of thesemiconductor switch 11.

(4) Advantages

The control circuit 12 according to the first example includes the firstdischarge path 21, the second discharge path 22, the first switch Q11,and the second switch Q12. The first discharge path 21 is connected tothe gate 11G of the semiconductor switch 11. The second discharge path22 is connected to the gate 11G of the semiconductor switch 11. Thesecond discharge path 22 enables a more rapid discharge than the firstdischarge path 21. The second switch Q12 may be turned ON and OFFseparately from the first switch Q11. The second switch Q12 is providedon the second discharge path 22 and selectively turns ON according tothe current variation rate of a principal current I_(DS) of thesemiconductor switch 11. Thus, this control circuit 12 may reduce asurge voltage applied to the semiconductor switch 11 while cutting downthe switching loss involved when the semiconductor switch 11 turns OFF.

The control circuit 12 drains the electric charge in the gate throughthe first discharge path 21 in the period from the point in time t2 tothe point in time t3 during which the principal current I_(DS) of thesemiconductor switch 11 decreases when the semiconductor switch 11 turnsOFF. This enables reducing the generation of the surge voltage due tothe parasitic inductance of a load circuit connected to thesemiconductor switch 11 and the current variation rate of the principalcurrent I_(DS). Also, the control circuit 12 drains the electric chargein the gate through the second discharge path 22, through which theelectric charge may be drained more rapidly than through the firstdischarge path 21, in the periods other than the period from the pointin time t2 to the point in time t3 (namely, the period from the point intime t1 to the point in time t2 and the period from the point in time t3to the point in time t4) while the semiconductor switch 11 turns OFF.This contributes to shortening the turn-off time. This allows thecontrol circuit 12 and the switch system 13 to reduce the chances ofextending the switching time and cut down the switching loss even if thesurge voltage is reduced by decreasing the absolute value of the currentvariation rate of the semiconductor switch 11.

In addition, in the control circuit 12, the first discharge path 21includes the gate resistor R_(G). This also enables, after a part of theelectric charge has been drained from the gate 11G of the semiconductorswitch 11 through the second discharge path 22, reducing the absolutevalue of the current variation rate of the principal current I_(DS) whenthe residual electric charge is drained from the gate 11G of thesemiconductor switch 11 through the first discharge path 21.

SECOND EXAMPLE

Next, a control circuit 12 a according to a second example and a switchsystem 13 a including the control circuit 12 a will be described withreference to FIG. 23 .

The control circuit 12 a according to the second example is almost thesame as the control circuit 12 according to the first example (see FIG.17 ) but includes, as the first switch Q11, an n-channel field effecttransistor Tr11 instead of the p-channel field effect transistor Tr1,which is a difference from the control circuit 12 according to the firstexample. In the following description of the control circuit 12 a andswitch system 13 a according to the second example, any constituentelement of this second example, having the same function as acounterpart of the control circuit 12 and switch system 13 according tothe first example described above, will be designated by the samereference numeral as that counterpart's, and description thereof will beomitted herein.

In the control circuit 12 a according to the second example, the firstswitch Q11 is an n-channel field effect transistor Tr11 provided on thesecond discharge path 22.

The n-channel field effect transistor Tr11 includes a gate, a source,and a drain. The field effect transistor Tr11 (hereinafter also referredto as a “first field effect transistor Tr11”) is a normally OFFn-channel MOSFET in the example illustrated in FIG. 23 . Meanwhile, thesecond switch Q12 is a diode D2 provided on the second discharge path22. The diode D2 includes an anode and a cathode.

In the control circuit 12 a, the drain of the first field effecttransistor Tr11 is connected to the gate 11G of the semiconductor switch11 and the source of the first field effect transistor Tr11 is connectedto the anode of the diode D2. The second discharge path 22 includes aninductor Ls which is connected in series to the diode D2. Thus, in thesecond discharge path 22, the first field effect transistor Tr11, thediode D2, and the inductor L1 are connected in series.

The control circuit 12 a further includes a series circuit of a resistorR11 and a third switch Q13. The resistor R11 has a first terminal and asecond terminal. The third switch Q13 is an n-channel field effecttransistor Tr3. The n-channel field effect transistor Tr3 includes agate, a source, and a drain. The field effect transistor Tr3(hereinafter also referred to as a “third field effect transistor Tr3”)is a normally OFF n-channel MOSFET in the example illustrated in FIG. 23. In the control circuit 12 a, the first terminal of the resistor R11 isconnected to the drain of the first field effect transistor Tr11 and thesecond terminal of the resistor R11 is connected to the drain of thethird field effect transistor Tr3. The source of the third field effecttransistor Tr3 is connected to the low-potential output terminal of thedriver 14 and the source 11S of the semiconductor switch 11. The gate ofthe third field effect transistor Tr3 is connected to the high-potentialoutput terminal of the driver 14. The gate of the first field effecttransistor Tr11 is connected to a node between the second terminal ofthe resistor R11 and the drain of the third transistor Tr3.

In the switch system 13 a, while a positive bias voltage is output fromthe driver 14 to between the gate 11G and source 11S of thesemiconductor switch 11, the semiconductor switch 11 is in ON state. Atthis time, in the control circuit 12 a, the third field effecttransistor Tr3 is in ON state and the first field effect transistor Tr11is in OFF state.

To turn the semiconductor switch 11 OFF, the switch system 13 a changesthe output voltage of the driver 14 from a positive bias voltage into 0V, for example, (or a negative bias voltage). As a result, in thecontrol circuit 12 a, the third field effect transistor Tr3 turns OFF,the first field effect transistor Tr11 turns ON, and therefore, theelectric charge in the gate 11G of the semiconductor switch 11 isdrained through the second discharge path 22.

Thereafter, in the control circuit 12 a, when the principal currentI_(DS) of the semiconductor switch 11 starts to decrease, electromotiveforce is generated between the first terminal and the second terminal ofthe inductor Ls due to the variation in the principal current I_(DS) tocause the diode D2 to turn OFF. As a result, the current flowing throughthe first switch Q11 (i.e., the first field effect transistor Tr11)decreases, and therefore, the gate current I_(G) (see FIG. 20 ) flowsthrough the gate resistor R_(G). That is to say, the electric charge inthe gate 11G of the semiconductor switch 11 starts to be drained throughthe first discharge path 21, instead of the second discharge path 22.Thus, the magnitude of the gate current I_(G) is determined by theresistance value of the gate resistor R_(G).

Thereafter, in the control circuit 12 a, when the principal currentI_(DS) of the semiconductor switch 11 becomes approximately equal tozero, no electromotive force is generated in the inductor Ls any longerand the second switch Q12 turns ON. Thus, the gate current I_(G) startsto flow through the second discharge path 22 instead of the firstdischarge path 21. That is to say, in the gate current I_(G), thecurrent I_(Q11) (see FIG. 21 ) flowing through the first switch Q11becomes dominant. Thus, the electric charge in the gate of thesemiconductor switch 11 is drained rapidly through the second dischargepath 22. Consequently, the gate 11G-source 11S voltage V_(GS) of thesemiconductor switch 11 decreases steeply to become approximately equalto zero.

The control circuit 12 a and the switch system 13 a according to thesecond example, as well as the control circuit 12 and switch system 13according to the first example, may reduce a surge voltage applied tothe semiconductor switch 11 while cutting down the switching lossinvolved when the semiconductor switch 11 turns OFF.

In addition, in the switch system 13 a according to the second example,a monolithic integrated circuit, in which the control circuit 12 aincluding the first field effect transistor Tr11 and the third fieldeffect transistor Tr3 and the semiconductor switch 11 are integratedtogether, may be easily provided by implementing each of the first fieldeffect transistor Tr11 and the third field effect transistor Tr3 as ann-channel GaN-based GIT.

THIRD EXAMPLE

Next, a control circuit 12 b according to a third example and a switchsystem 13 b including the control circuit 12 b will be described withreference to FIG. 24 .

The control circuit 12 b according to the third example is almost thesame as the control circuit 12 according to the first example (see FIG.17 ) but includes, as the second switch Q12, a normally ON n-channelfield effect transistor Tr2 instead of the diode D2, which is adifference from the control circuit 12 according to the first example.In the following description of the control circuit 12b and switchsystem 13 b according to the third example, any constituent element ofthis third example, having the same function as a counterpart of thecontrol circuit 12 and switch system 13 according to the first exampledescribed above, will be designated by the same reference numeral asthat counterpart's, and description thereof will be omitted herein.

In the control circuit 12 b, the first switch Q11 is a p-channel fieldeffect transistor Tr1 provided on the second discharge path 22 and thesecond switch Q12 is a normally ON n-channel field effect transistor Tr2provided on the second discharge path 22. The second discharge path 22includes an inductor Ls which is connected in series to the n-channelfield effect transistor Tr2. In the second discharge path 22, theinductor Ls is connected to the source 11S of the semiconductor switch11.

The normally ON n-channel field effect transistor Tr2 includes a gate, asource, and a drain. In the example illustrated in FIG. 24 , the fieldeffect transistor Tr2 is a normally ON n-channel GaN-based GIT.

The drain of the field effect transistor Tr2 is connected to the drainof the field effect transistor Tr1. The source of the field effecttransistor Tr2 is connected to the first terminal of the inductor Ls.The gate of the field effect transistor Tr2 is connected to the secondterminal of the inductor Ls. Thus, the gate of the field effecttransistor Tr2 is connected to the low-potential output terminal of thedriver 14 and the source 11S of the semiconductor switch 11.

In the switch system 13 b, while a positive bias voltage is output fromthe driver 14 to between the gate 11G and source 11S of thesemiconductor switch 11, the semiconductor switch 11 is in ON state. Atthis time, in the control circuit 12 b, the field effect transistor Tr11is in OFF state.

To turn the semiconductor switch 11 OFF, the switch system 13 b changesthe output voltage of the driver 14 from a positive bias voltage into 0V, for example, (or a negative bias voltage). As a result, in thecontrol circuit 12 b, the field effect transistor Tr1 turns ON, andtherefore, the electric charge in the gate 11G of the semiconductorswitch 11 is drained through the second discharge path 22.

Thereafter, in the control circuit 12 b, when the principal currentI_(DS) of the semiconductor switch 11 starts to decrease, electromotiveforce is generated between the first terminal and the second terminal ofthe inductor Ls due to the variation in the principal current I_(DS) tocause the field effect transistor Tr2 to turn OFF. As a result, thecurrent flowing through the field effect transistor Tr1 decreases, andtherefore, the gate current I_(G) (see FIG. 20 ) flows through the gateresistor R_(G). That is to say, the electric charge in the gate 11G ofthe semiconductor switch 11 starts to be drained through the firstdischarge path 21, instead of the second discharge path 22. Thus, themagnitude of the gate current I_(G) is determined by the resistancevalue of the gate resistor R_(G).

Thereafter, in the control circuit 12 b, when the principal currentI_(DS) of the semiconductor switch 11 becomes approximately equal tozero, no electromotive force is generated in the inductor Ls any longerand the second switch Q12 turns ON. Thus, the gate current I_(G) startsto flow through the second discharge path 22 instead of the firstdischarge path 21. That is to say, in the gate current I_(G), thecurrent I_(Q1) (see FIG. 21 ) flowing through the first switch Q11becomes dominant Thus, the electric charge in the gate of thesemiconductor switch 11 is drained rapidly through the second dischargepath 22. Consequently, the gate 11G-source 11S voltage V_(GS) of thesemiconductor switch 11 decreases steeply to become approximately equalto zero.

The control circuit 12 b and the switch system 13 b according to thethird example, as well as the control circuit 12 and switch system 13according to the first example, may reduce a surge voltage applied tothe semiconductor switch 11 while cutting down the switching lossinvolved when the semiconductor switch 11 turns OFF.

In the example illustrated in FIG. 24 , the field effect transistor Tr2is a normally ON n-channel GaN-based GIT as described above. However,this is only an example and should not be construed as limiting.Alternatively, the field effect transistor Tr2 may also be a normally ONn-channel MOSFET.

FOURTH EXAMPLE

Next, a switch system 13 e according to a fourth example will bedescribed with reference to FIG. 25 .

The switch system 13 e according to this fourth example includes twosemiconductor switches 11 and two control circuits 12 of the switchsystem 13 according to the first example, which is a difference from theswitch system 13 according to the first example. In the followingdescription of the switch system 13 e according to the fourth example,any constituent element, having the same function as a counterpart ofthe switch system 13 according to the first example described above,will be designated by the same reference numeral as that counterpart's,and description thereof will be omitted herein.

In the switch system 13 e, the two semiconductor switches 11 areconnected in series and the two control circuits 12 are associated oneto one with the two semiconductor switches 11.

In the switch system 13 e according to the fourth example, therespective drains 11D of the two semiconductor switches 11 are connectedto each other.

In this switch system 13 e, the polarity of the electromotive forcegenerated in one of the two inductors Ls due to a variation in currentis different from that of the electromotive force generated in the otherinductor Ls due to the variation in current. In the inductor Lsconnected to the source 11S of one of the two semiconductor switches 11,electromotive force is generated to cause the cathode of the diode D2 tohave a higher potential than the source 11S. In the inductor Lsconnected to the source 11S of the other semiconductor switch 11,electromotive force is generated to cause the cathode of the diode D2 tohave a lower potential than the source 11S. Thus, the switch system 13 emay reduce a surge voltage applied to the semiconductor switch 11 whilecutting down the switching loss involved when the semiconductor switch11, associated with the inductor Ls in which the electromotive force isgenerated to cause the cathode of the diode D2 to have a higherpotential than the source 11S, turns OFF.

The switch system 13 e according to the fourth example, as well as theswitch system 13 according to the first example, may reduce a surgevoltage applied to the semiconductor switch 11 while cutting down theswitching loss involved when the semiconductor switch 11 turns OFF.

FIFTH EXAMPLE

Next, a switch system 13 f according to a fifth example will bedescribed with reference to FIG. 26 .

The switch system 13 f according to this fifth example includes twosemiconductor switches 11 and two control circuits 12, which is adifference from the switch system 13 according to the first example. Inthe following description of the switch system 13 f according to thefifth example, any constituent element, having the same function as acounterpart of the switch system 13 according to the first exampledescribed above, will be designated by the same reference numeral asthat counterpart's, and description thereof will be omitted herein.

In the switch system 13 f, the two semiconductor switches 11 areconnected in series and the two control circuits 12 are associated oneto one with the two semiconductor switches 11.

In the switch system 13 f, the respective sources 11S of the twosemiconductor switches 11 are connected via the respective inductors Lsof the two control circuits 12. Each of the two diode D2 of the twocontrol circuits 12 is connected to the inductor Ls of an associated oneof the two control circuits 12 via the inductor Ls of the controlcircuit 12 other than the associated one of the two control circuits 12.

The switch system 13 f according to the fifth example, as well as theswitch system 13 according to the first example, may reduce a surgevoltage applied to the semiconductor switch 11 while cutting down theswitching loss involved when the semiconductor switch 11 turns OFF.

The switch system 13 f according to the fifth example includes twodrivers 14 for the two control circuits 12 and the respectivelow-potential output terminals of the two drivers 14 are connected toeach other. However, this is only an example and should not be construedas limiting. Alternatively, a single driver 14 may also be used incommon for the two control circuits 12.

SIXTH EXAMPLE

Next, a switch system 13 g according to a sixth example will bedescribed with reference to FIG. 27 .

The switch system 13 g according to this sixth example includes twosemiconductor switches 11 and two control circuits 12, which is adifference from the switch system 13 according to the first example. Inthe following description of the switch system 13 g according to thesixth example, any constituent element, having the same function as acounterpart of the switch system 13 according to the first exampledescribed above, will be designated by the same reference numeral asthat counterpart's, and description thereof will be omitted herein.

In the switch system 13 g, the two semiconductor switches 11 areconnected in series and the two control circuits 12 are associated oneto one with the two semiconductor switches 11.

In the switch system 13 g, the respective sources 11S of the twosemiconductor switches 11 are connected via the respective inductors Lsof the two control circuits 12. In the switch system 13 g, therespective sources 11S of the two semiconductor switches 11 areconnected to each other. In the switch system 13 g, a node N13 betweenthe respective inductors Ls of the two control circuits 12 and a nodeN14 between the respective cathodes of the diodes D2 of the two controlcircuits 12 are connected to each other.

The switch system 13 g according to the sixth example, as well as theswitch system 13 according to the first example, may reduce a surgevoltage applied to the semiconductor switch 11 while cutting down theswitching loss involved when the semiconductor switch 11 turns OFF.

SEVENTH EXAMPLE

Next, a switch system 13 h according to a seventh example will bedescribed with reference to FIG. 28 .

The switch system 13 h according to this seventh example includes asemiconductor switch 11 h instead of the semiconductor switch 11 of theswitch system 13 e according to the fourth example, which is adifference from the switch system 13 e according to the fourth example.The semiconductor switch 11 h is a dual-gate bidirectional switch havingtwo gates 11G and two sources 11S.

In the semiconductor switch 11 h, the two gates 11G and the two sources11S correspond one to one to each other. In the following description,one of the two gates 11G will be hereinafter referred to as a “firstgate 111G” and the other gate 11G as a “second gate 112G” for the sakeof convenience of description. In the same way, out of the two sources11S, the source 11S corresponding to the first gate 111G will behereinafter referred to as a “first source 111S” and the source 11Scorresponding to the second gate 112G will be hereinafter referred to asa “second source 112S.”

In the following description, the semiconductor switch 11 h will bedescribed briefly and then the switch system 13 h will be described.

The semiconductor switch 11 h is a type of GaN-based GIT. Thesemiconductor switch 11 h includes, for example, a substrate, a bufferlayer, a first nitride semiconductor layer, a second nitridesemiconductor layer, a first source electrode, a first gate electrode, asecond gate electrode, a second source electrode, a first p-type layer,and a second p-type layer. The buffer layer is formed on the substrate.The first nitride semiconductor layer is formed on the buffer layer. Thesecond nitride semiconductor layer is formed on the first nitridesemiconductor layer. The first source electrode, the first gateelectrode, the second gate electrode, and the second source electrodeare formed on the second nitride semiconductor layer. The first p-typelayer is interposed between the first gate electrode and the secondnitride semiconductor layer. The second p-type layer is interposedbetween the second gate electrode and the second nitride semiconductorlayer. In the semiconductor switch 11, the first source 111S includesthe first source electrode. The first gate 111G includes the first gateelectrode and the first p-type layer. The second gate 112G includes thesecond gate electrode and the second p-type layer. The second source112S includes the second source electrode. The substrate is a siliconsubstrate, for example. The buffer layer is an undoped GaN layer, forexample. The first nitride semiconductor layer is, for example, anundoped GaN layer. The second nitride semiconductor layer is, forexample, an undoped AlGaN layer. Each of the first p-type layer and thesecond p-type layer is, for example, a p-type AlGaN layer. Each of thebuffer layer, the first nitride semiconductor layer, and the secondnitride semiconductor layer may include impurities such as Mg, H, Si, C,and O to be inevitably contained during their growing process bymetal-organic vapor phase epitaxy (MOVPE), for example.

In the semiconductor switch 11 h, the second nitride semiconductor layerforms, along with the first nitride semiconductor layer, aheterojunction portion. In the first nitride semiconductor layer, atwo-dimensional electron gas has been generated in the vicinity of theheterojunction portion. A region including the two-dimensional electrongas (hereinafter referred to as a “two-dimensional electron gas layer”)may also serve as an n-channel layer (electron conduction layer).

Also, in the following description, a state where a voltage equal to orhigher than a first threshold voltage (of 1.3 V, for example) is notapplied between the first gate 111G and the first source 111S with thefirst gate 111G having the higher potential will be hereinafter referredto as a “state where the first gate 111G is OFF.” Also, a state where avoltage equal to or higher than the first threshold voltage is appliedbetween the first gate 111G and the first source 111S with the firstgate 111G having the higher potential will be hereinafter referred to asa “state where the first gate 111G is ON.” Furthermore, a state where avoltage equal to or higher than a second threshold voltage (of 1.3 V,for example) is not applied between the second gate 112G and the secondsource 112S with the second gate 112G having the higher potential willbe hereinafter referred to as a “state where the second gate 112G isOFF.” Also, a state where a voltage equal to or higher than the secondthreshold voltage is applied between the second gate 112G and the secondsource 112S with the second gate 112G having the higher potential willbe hereinafter referred to as a “state where the second gate 112G isON.”

This semiconductor switch 11 h includes the first p-type layer and thesecond p-type layer described above, thus implementing a normally OFFtransistor.

The semiconductor switch 11 h may be switched from one of abidirectionally ON state, a bidirectionally OFF state, a first diodestate, or a second diode state to another depending on the combinationof a first gate voltage applied to the first gate 111G and a second gatevoltage applied to the second gate 112G. The first gate voltage is avoltage applied between the first gate 111G and the first source 111S.The second gate voltage is a voltage applied between the second gate112G and the second source 112S. The bidirectionally ON state is a statewhere an electric current is allowed to pass bidirectionally (i.e., in afirst direction and a second direction opposite from the firstdirection). The bidirectionally OFF state is a state where an electriccurrent is blocked bidirectionally. The first diode state is a statewhere an electric current is allowed to pass in the first direction. Thesecond diode state is a state where an electric current is allowed topass in the second direction.

In a state where the first gate 111G is ON and the second gate 112G isON, the semiconductor switch 11h turns into the bidirectionally ONstate. In a state where the first gate 111G is OFF and the second gate112G is OFF, the semiconductor switch 11 h turns into thebidirectionally OFF state. In a state where the first gate 111G is OFFand the second gate 112G is ON, the semiconductor switch 11 h turns intothe first diode state. In a state where the first gate 111G is ON andthe second gate 112G is OFF, the semiconductor switch 11 h turns intothe second diode state.

In the switch system 13 h, the first discharge path 21 and the seconddischarge path 22 in one of the two control circuits 12 are connected tothe first gate 111G, which is one of the two gates 11G, and the firstdischarge path 21 and the second discharge path 22 in the other controlcircuit 12 are connected to the second gate 112G, which is the other ofthe two gates 11G. In the switch system 13 h, the inductor Ls of one ofthe two control circuits 12 is connected to the first source 111S,corresponding to the first gate 111G, out of the two sources 11S, andthe inductor Ls of the other control circuit 12 is connected to thesecond source 112S, corresponding to the second gate 112G, out of thetwo sources 11S.

The switch system 13 h according to the seventh example, as well as theswitch system 13 according to the first example, may reduce a surgevoltage applied to the semiconductor switch 11h while cutting down theswitching loss involved when the semiconductor switch 11 h turns OFF.

Note that the first to seventh examples described above are onlyexemplary ones of various examples of the present disclosure and shouldnot be construed as limiting. Rather, the first through seventh examplesmay be readily modified in various manners depending on a design choiceor any other factor without departing from the scope of the presentdisclosure.

Also, the p-type layer of the semiconductor switch 11 of the switchsystem 13 h does not have to be the p-type AlGaN layer but may be, forexample, a p-type GaN layer or a p-type metal-oxide semiconductor layeras well. The p-type metal-oxide semiconductor layer may be, for example,an NiO layer. The NiO layer may contain, as an impurity, at least onealkali metal selected from the group consisting of lithium, sodium,potassium, rubidium, and cesium. The NiO layer may also contain atransition metal such as silver or copper which becomes univalent whenadded as an impurity, for example. This statement for the p-type layerof the semiconductor switch 11 also applies to each of the first p-typelayer and the second p-type layer of the semiconductor switch 11 h ofthe switch system 13 h.

Each of the semiconductor switch 11 and the semiconductor switch 11 hmay include one or more nitride semiconductor layers between the bufferlayer and the first nitride semiconductor layer. Furthermore, the bufferlayer does not have to have a single-layer structure but may also have,for example, a superlattice structure.

Furthermore, in each of the semiconductor switch 11 and thesemiconductor switch 11 h, the substrate does not have to be a siliconsubstrate but may also be, for example, a GaN substrate, an SiCsubstrate, or a sapphire substrate.

ASPECTS

The first to seventh examples and their variations described above maybe specific implementations of the following aspects of the presentdisclosure.

A control circuit (12; 12 a; 12 b) according to a first aspect is acontrol circuit for controlling a semiconductor switch (11; 11 h)including a gate (11G) and a source (11S) corresponding to the gate(11G). The control circuit (12; 12 a; 12 b) includes a first dischargepath (21), a second discharge path (22), a first switch (Q11), and asecond switch (Q12). The first discharge path (21) is connected to thegate (11G) of the semiconductor switch (11; 11 h). The second dischargepath (22) is connected to the gate (11G) of the semiconductor switch(11; 11 h). The second discharge path (22) enables a more rapiddischarge than the first discharge path (21). The second switch (Q12)may be turned ON and OFF separately from the first switch (Q11). Thesecond switch (Q12) is provided on the second discharge path (22) andturns ON according to a current variation rate.

The control circuit (12; 12 a; 12 b) according to the first aspect mayreduce a surge voltage applied to the semiconductor switch (11; 11 h)while cutting down the switching loss involved when the semiconductorswitch (11; 11 h) turns OFF.

In a control circuit (12; 12 a; 12 b) according to a second aspect,which may be implemented in conjunction with the first aspect, the firstswitch (Q11) is provided on the second discharge path (22).

The control circuit (12; 12 a; 12 b) according to the second aspectallows electricity to be selectively discharged, according to the stateof the first switch (Q11), through the second discharge path (22).

In a control circuit (12; 12 a; 12 b) according to a third aspect, whichmay be implemented in conjunction with the first or second aspect, thefirst switch (Q11) turns ON when the semiconductor switch (11; 11 h)turns OFF.

The control circuit (12; 12 a; 12 b) according to the third aspectallows draining electric charge from the gate (11G) of the semiconductorswitch (11; 11 h) via the first switch (Q11) when the semiconductorswitch (11; 11 h) turns OFF.

In a control circuit (12) according to a fourth aspect, which may beimplemented in conjunction with any one of the first to third aspects,the first switch (Q11) is a p-channel field effect transistor (Tr1)provided on the second discharge path (22). The second switch (Q12) is adiode (D2) provided on the second discharge path (22). The seconddischarge path (22) includes an inductor (Ls) connected in series to thediode (D2). In the second discharge path (22), the inductor (Ls) isconnected to the source (11S) of the semiconductor switch (11; 11 h).

The control circuit (12) according to the fourth aspect enables reducingthe chances of causing voltage drop in each of the first switch (Q11)and the second switch (Q12).

In a control circuit (12 a) according to a fifth aspect, which may beimplemented in conjunction with any one of the first to third aspects,the first switch (Q11) is an n-channel field effect transistor (Tr11)provided on the second discharge path (22). The second switch (Q12) is adiode (D2) provided on the second discharge path (22). The seconddischarge path (22) includes an inductor (Ls) connected in series to thediode (D2). In the second discharge path (22), the inductor (Ls) isconnected to the source (11S) of the semiconductor switch (11).

The control circuit (12 a) according to the fifth aspect enablesreducing the chances of causing voltage drop in each of the first switch(Q11) and the second switch (Q12).

In a control circuit (12 b) according to a sixth aspect, which may beimplemented in conjunction with any one of the first to third aspects,the first switch (Q11) is a p-channel field effect transistor (Tr1)provided on the second discharge path (22). The second switch (Q12) is anormally ON n-channel field effect transistor (Tr2) provided on thesecond discharge path (22). The second discharge path (22) includes aninductor (Ls) connected in series to the n-channel field effecttransistor (Tr2). In the second discharge path (22), the inductor (Ls)is connected to the source (11S) of the semiconductor switch (11).

The control circuit (12 b) according to the sixth aspect enablesreducing the chances of causing voltage drop in each of the first switch(Q11) and the second switch (Q12).

In a control circuit (12) according to a seventh aspect, which may beimplemented in conjunction with the first or second aspect, the firstswitch (Q11) turns ON when the semiconductor switch (11) turns ON.

In a control circuit (12; 12 a; 12 b) according to an eighth aspect,which may be implemented in conjunction with any one of the first toseventh aspects, the first discharge path (21) includes a gate resistor(R_(G)) connected to the gate (11G) of the semiconductor switch (11; 11h). The second discharge path (22) is connected to the gate (11G) of thesemiconductor switch (11; 11 h) not via the gate resistor (R_(G)).

The control circuit (12; 12 a; 12 b) according to the eighth aspectenables changing the current variation rate of the principal current(I_(DS)) of the semiconductor switch (11; 11 h) by changing theresistance value of the gate resistor (R_(G)).

A switch system (13; 13 a; 13 b; 13 e; 13 g; 13 h) according to a ninthaspect includes the control circuit (12; 12 a; 12 b) according to anyone of the first to eighth aspects; and a semiconductor switch (11; 11h).

The switch system (13; 13 a, 13 b; 13 e; 13 g; 13 h) according to theninth aspect may reduce a surge voltage applied to the semiconductorswitch (11; 11 h) while cutting down the switching loss involved whenthe semiconductor switch (11; 11 h) turns OFF.

A switch system (13 e; 13 f; 13 g) according to a tenth aspect, whichmay be implemented in conjunction with the ninth aspect, includes twosemiconductor switches (11) and two control circuits (12). In the switchsystem (13 e; 13 f; 13 g), the two semiconductor switches (11) areconnected in series. The two control circuits (12) are associated one toone with the two semiconductor switches (11).

The switch system (13 e; 13 f; 13 g) according to the tenth aspectenables changing the current variation rate of the principal current(I_(DS)) of the semiconductor switch (11) by changing the resistancevalue of the gate resistor (R_(G)) with respect to each of the twosemiconductor switches (11).

In a switch system (13 e) according to an eleventh aspect, which may beimplemented in conjunction with the tenth aspect, each of the twosemiconductor switches (11) includes a drain (11D) corresponding to thegate (11G). In the switch system (13 e), the respective drains (11D) ofthe two semiconductor switches (11) are connected to each other.

In a switch system (13 f) according to a twelfth aspect, which may beimplemented in conjunction with the tenth aspect, in each of the twocontrol circuits (12), the first switch (Q11) is a p-channel fieldeffect transistor (Tr1) provided on the second discharge path (22). Ineach of the two control circuits (12), the second switch (Q12) is adiode (D2) provided on the second discharge path (22). In each of thetwo control circuits (12), the second discharge path (22) includes aninductor (Ls) connected in series to the diode (D2). The inductor (Ls)is connected to the source (11S) of the semiconductor switch (11). Inthe switch system (13 f), the respective sources (11S) of the twosemiconductor switches (11) are connected to each other via theinductors (Ls) of the two control circuits (12). Each of the diodes (D2)of the two control circuits (12) is connected to the inductor (Ls) of anassociated control circuit (12) via the inductor (Ls) of the other,non-associated control circuit (12) out of the two control circuits(12).

In a switch system (13 g) according to a thirteenth aspect, which may beimplemented in conjunction with the tenth aspect, in each of the twocontrol circuits (12), the first switch (Q11) is a p-channel fieldeffect transistor (Tr1) provided on the second discharge path (22). Ineach of the two control circuits (12), the second switch (Q12) is adiode (D2) provided on the second discharge path (22). In each of thetwo control circuits (12), the second discharge path (22) includes aninductor (Ls) connected in series to the diode (D2). The inductor (Ls)is connected to the source (11S) of its associated semiconductor switch(11). In the switch system (13 g), the respective sources (11S) of thetwo semiconductor switches (11) are connected to each other via theinductors (Ls) of the two control circuits (12). In the switch system(13 g), a node (N13) between the respective inductors (Ls) of the twocontrol circuits (12) and a node (N14) between the respective cathodesof the diodes (D2) of the two control circuits (12) are connected toeach other.

The switch system (13 g) according to the thirteenth aspect allows asingle driver (14) to be used in common for the two control circuits(12).

In a switch system (13 h) according to a fourteenth aspect, which may beimplemented in conjunction with the ninth aspect, the semiconductorswitch (11) is a dual-gate bidirectional switch including two gates(11G) and two sources (11S). The switch system (13 h) includes twocontrol circuits (12). In the switch system (13 h), one control circuit(12) out of the two control circuits (12) is connected to a first gate(111G), which is one gate (11G) out of the two gates (11G), and theother control circuit (12) is connected to a second gate (112G), whichis the other gate (11G) out of the two gates (11G).

REFERENCE SIGNS LIST

1, 1 k Switching Element

5 Circuit Element

10, 10 a, 10 b, 10 c, 10 d, 10 e 1, 10 e 2, 10 f 1, 10 f 2, 10 g 1, 10 g2, 10 h, 10 i, 10 j Control Circuit

100, 100 a, 100 b, 100 c, 100 d, 100 e, 100 f, 100 g, 100 h, 100 i, 100j, 100 k Switch Device

D1 Drain

Di1 Diode

Di2 Protective Diode

Di3 Protective Diode

Dis Diode

G1 Gate

L1 Inductor

P0 Reference Potential Point

R1 Resistor

S1, S2 Source

11 Semiconductor Switch

11D Drain

11G Gate

111G First Gate

112G Second Gate

11S Source

111S First Source

112S Second Source

12, 12 a, 12 b Control Circuit

21 First Discharge Path

22 Second Discharge Path

13, 13 a, 13 b, 13 e, 13 f, 13 g, 13 h Switch System

14 Driver

N11 Node

N12 Node

N13 Node

N14 Node

D2 Diode

Ls Inductor

R_(G) Gate Resistor

Tr1 p-Channel Field Effect Transistor

Tr11 Field Effect Transistor

Tr2 Normally ON n-Channel Field Effect Transistor

1. A control circuit configured to control a switching element, theswitching element including a gate and a source corresponding to thegate, the control circuit comprising: an inductor connected between thegate and the source of the switching element; a circuit elementconnected in series to the inductor between the gate and the source andconfigured to allow an electric current to flow therethrough in responseto generation of electromotive force in the inductor; and a resistorconnected in parallel to the inductor and the circuit element betweenthe gate and the source.
 2. The control circuit of claim 1, wherein thecircuit element includes a capacitor.
 3. The control circuit of claim 1,wherein the circuit element includes a diode.
 4. The control circuit ofclaim 1, wherein the circuit element includes a resistor.
 5. The controlcircuit of claim 1, wherein when the switching element turns OFF, anelectric current flowing through the source of the switching elementdecreases to generate electromotive force in the inductor, cause anelectric current corresponding to the electromotive force to flowthrough the circuit element and the resistor, and thereby cause anincrease in a potential at a reference potential point included in apath between a point of connection where the circuit element and theresistor are connected together and the gate, and an amount of anelectric current flowing through the path from the gate varies accordingto a potential difference between a potential at the gate and thepotential at the reference potential point.
 6. The control circuit ofclaim 1, further comprising a protective diode, the protective diodeincluding an anode and a cathode, having the anode connected to a pointof connection between the circuit element and the resistor, and havingthe cathode connected to the gate of the switching element.
 7. Thecontrol circuit of claim 1, further comprising a protective diode, theprotective diode including an anode and a cathode, having the anodeconnected between the source of the switching element and a node locatedbetween the inductor and the resistor, and having the cathode connectedto the gate of the switching element.
 8. The control circuit of claim 1,further comprising: a first terminal connected to the switching elementat a point, located opposite from the source, of the switching element;a second terminal connected to the inductor at a point, located oppositefrom the switching element, of the inductor; a second inductor connectedbetween a first node and the circuit element, the first node beinglocated between a first inductor serving as the inductor and the secondterminal; a voltage clamping element connected in parallel to theswitching element, the first inductor, and the second inductor; and athird inductor connected between a second node and the voltage clampingelement, the second node being located between the second inductor andthe circuit element, wherein no electric current flows through the thirdinductor while the switching element is in ON state.
 9. A switch devicecomprising: the control circuit of claim 1; and the switching element.10. The switch device of claim 9, wherein the switch device includes twoswitching elements, each of the two switching elements being theswitching element, the switch device includes two control circuits, eachof the two control circuits being the control circuit, the two switchingelements are connected in series, and the two control circuits areassociated one to one with the two switching elements.
 11. The switchdevice of claim 10, wherein each of the two switching elements includesa drain corresponding to the gate, and the respective drains of the twoswitching elements are connected to each other.
 12. The switch device ofclaim 9, wherein the switching element, is a dual-gate bidirectionalswitch including two gates and two sources, each of the two gates beingthe gate, each of the two sources being the source, the switch deviceincludes two control circuits, each of the two control circuits beingthe control circuit, and one control circuit out of the two controlcircuits is connected to a gate, associated with the one controlcircuit, out of the two gates of the bidirectional switch, and the othercontrol circuit out of the two control circuits is connected to a gate,associated with the other control circuit, out of the two gates of thebidirectional switch.
 13. The switch device of claim 10, wherein therespective sources of the two switching elements are connected to eachother.